UK optical discs manufacturer, located in Blackburn, and operating under the legal name of PDO Discs Ltd. from December 1986 until March 1996.
(Some items released in late 1996 are known to still display a Made in U.K. by PDO credit, possibly due to the use of old stock by the successor company, PMDC.)

Regularly credited in CD matrices with (capitalized) "Made in U.K." or "Made in U.K. by PDO" (or P.D.O.), PDO Discs Ltd. was the UK branch of PDO (Philips DuPont Optical) based in Blackburn, Lancashire.

Originally established in 1980 by Philips Electronics to manufacture Laser Discs, by December 1986 the plant also manufactured CDs, CDRs, and CDVideos. In April 1996 control was handed over to PMDC, UK after the plant was sold to PolyGram.

PDO UK had a strictly sequential numbering system, unique to this plant:
Between 1987 and June 1989 the pattern consisted of 6+2 digits:
XYYYYZ NN (e.g. 134231 01)

where X = 1 (first disc) or 2 (second disc for 2xCD editions) and Z = number of discs in the set
YYYY = sequential manufacturing number,
NN = 01 up to 09 or even higher (potentially the glass master number).

From July 1989 onwards, the system was enhanced to 8+2 digits, with optional symbol presumably identifying the LBR:
XYYYYYYZ NN (e.g. 10133491 01 or 20133491 01 %)

where X = 1 (first disc) or 2 (second disc for 2xCD editions),
YYYYYY = sequential manufacturing number,
NN = 01 to 09, 10, 11, etc. (potentially the glass master number).

The pattern was valid for all discs types (CD, CD-Single, CD-Mini, or CDV).

Only add XYYYYZ and XYYYYYYZ as LCCN catalog number.
For 2xCD editions add Made By - PDO, UK twice with one number each.

Note: The pattern can also be found on other UK plants such as Mayking or VDC. In such cases, PDO should be added for Glass Mastered At plus sequential number.
See an example here.

The Blackburn factory changed hands several more times, being operated by Deluxe (7) and eventually being acquired by EDC and operating as EDC Blackburn Ltd until its closure on December 31, 2009 with all operations moving to EDC's main European plant in Hannover, Germany.

Due to manufacturing problems at the UK PDO plant, many releases from the late 80s and early 90s eventually suffered from CD bronzing (or "disc rot"), and are rendered unplayable. When the problem became known in the early 1990s, PDO (and later its successors) offered to replace any discs thus affected, and pledged to re-press new CDs to meet the demand.
However, the helpline was discontinued in 2006, and defective CDs are now no longer replaced, even though some of the affected record labels continue to offer replacements.
Such repressings can be identified by a matrix number starting with 198xxxxx, where the 198 usually replaces the first 1 to 3 numbers of the original number (see this example, where 198 replaced the 1).

A list of affected discs (open for additions):

A HOMEBOY A HIPPIE & A FUNKYDRED, temple of love, PZCD241 10210491 01%
BIG AUDIO DYNAMITE Other 99 CDBAAD5 125111 01 % MADE IN THE U.K. BY PDO
BOYD RICE AND FRIENDS, music martinis and misantrophy, BADVC CD 1969 10120881 02 &
BROS I Quit CD ATOM 130781 03 % MADE IN THE U.K. BY PDO
COIL, stolen and contaminated songs, LOCICD4 10204341 01 &
COIL, unnatural history, LOCI CD 2 10102411 01%
CURRENT 93, christ and the pale queens mighty in sorrow, MAL 666CD 10098921 04 %
CURRENT 93, crooked crosses for the nodding god, UD 033 CD 10098881 01 %
CURRENT 93, dawn, DURTRO 002 CD 10120871 04 &
CURRENT 93, earth covers earth, DURTROO12CD 10207781 01 %
CURRENT 93, emblems: the menstrual years DISK1, DURTRO016 10227031 06 %
CURRENT 93, emblems: the menstrual years DISK2, DURTRO016 20227031 04 %
CURRENT 93, hitler as kalki, DURTRO014CD 10220071 02 %
CURRENT 93, imperium, DURTR0008CD 10168921 01 %
CURRENT 93, island, DURTROOO6CD 10164511 01 %
CURRENT 93, live at bar maldoror, DURTRO 001CD 10120861 02 &
CURRENT 93, looney runes, DURTRO004 10137251 01 &
CURRENT 93, nature unveiled, DURTRO009CD 10176441 01%
CURRENT 93, thunder perfect mind, DURTRO011CD 10180591 01 &
DEATH IN JUNE, oh how we laughed, EYAS1013 10098551 04%
DEATH IN JUNE, the cathedral of tears, BADVCCD34 10158031 01 %
FUSE, dimension intrusion, WARPCD12, 10233041 01 %
GARY NUMAN Isolate (The Numa Years) NUMACD1008 10180471 01 %
GARY NUMAN Ghost NUMACD1007 10203291 01 & MADE IN U.K. BY PDO / NUMACD1007 20203291 01 & MADE IN U.K. BY PDO
GARY NUMAN Machine + Soul NUVCD124 10195981 01 % MADE IN U.K. BY PDO
GARY NUMAN Machine + Soul NUCD2 24 10195991 01 % MADE IN U.K. BY PDO
LAIBACH, nova akropola, CDMRED 67 129121 05 %
LARD, the last temptation of reid, VIRUS84CD 10128371 02 &
MARC ALMOND AND THE WILLING SINNERS, vermine in ermine, 822 833 2 10186431 01 %
MARC AND THE MAMBAS, torment and toreros, 8128722 10181461 02 &, 8128722 20181461 01 &
MARC AND THE MAMBAS, untitled, 510289 2 10181451 01%
NEW ORDER, blue monday-95, NCDJ710365241 01^&
NURSE WITH WOUND, a sucked orange, UD 032 CD 10089281 01 %
NURSE WITH WOUND, chance meeting on a dissecting table..., UDOOCD 10129341 01 &
NURSE WITH WOUND, merzbild schwet, UDO4CD 10129351 01 &
NURSE WITH WOUND, the ladies home tickler, UD01CD 10129331 02 &
NURSE WITH WOUND, to the quiet man from a tony girl, UDO3CD 10129381 01 &
PET SHOP BOYS, left to my own devices, 203080-2 13371 01%
PSYCHIC TV, dreams less sweet, SBZCD 011 10202691 01%
RHYTHMATIC, wind me up, NWKCD25 10159351 02 %
SOFT CELL, non stop ecstatic dancing, 510 295 2 10181481 01 %
SOL INVICTUS, lex talionis, TURSA001CD 10129171 06 %
TERENCE TRENT D'ARBY This Side Of Love TRENTC5 10097521 01 &
TEST DEPT, pax brittanica, MOP6CD 10151191 01 %
TEST DEPT./BRITH GOF, gododdin, MOP4CD 10092701 01%
THE CLASH Rock The Casbah 6568142 10148091 02 & MADE IN U.K.
ULTRAVOX Ultravox! IMCD146 10195371 01
ULTRAVOX Ha! Ha! Ha! IMCD147 10195401 01 % MADE IN U.K. BY PDO
ULTRAVOX Systems Of Romance IMCD148 10195921 01 % MADE IN U.K. BY PDO
VARIOUS Some Bizzare Album, 510 297 2 10181491 01 & MADE IN U.K. BY PDO
VARIOUS Steamin! Hardcore '92 JARCD1 10172491 03 % MADE IN U.K.
VARIOUS Joy!! A Ché Compilation CD!