A DC10-30 FERRY FLT OVERSHOOTS ITS ASSIGNED ALT DURING A TRANSITION ALT PROCEDURAL CLB IN FOREIGN AIRSPACE.

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GATWICK ALTIMETER 987 MB. AFTER CLRNC FROM 4000 FT TO FL100; A FREQ CHANGE WAS GIVEN. THE NEW FREQ WAS BLOCKED AND CONTINUOUS ATTEMPTS AT ESTABLISHING CONTACT WERE THWARTED. THIS CAUSED A MISSED ALTIMETER CHANGEOVER AT 6000 FT. A TA WAS ISSUED OF TFC AT FL110; 12 O'CLOCK. APCHING 10000 FT; WE RECOGNIZED OUR ALTIMETER SETTING; RESET ALTIMETERS AND BEGAN A CORRECTION TO FL100. TCASII ISSUED A TA; THEN RA. CLOSEST ALT ON TCASII NOTED WAS 700 FT AND 1/2 MI OFFSET. NO OTHER MENTION OF THE CONFLICT WAS MADE. LOW ALT ALTIMETER CHANGEOVERS THAT OCCUR AT BUSY DEP TIMES ARE DANGEROUS. SUPPLEMENTAL INFO FROM ACN 430379: ATC CALLED AS THE ALT EXCURSION OCCURRED AND ASKED US TO CHK OUR ALTIMETER SETTING AND ADVISED US THAT WE WERE CLOSE TO THE OTHER ACFT. WE WERE TOO FOCUSED ON MAKING RADIO CONTACT. THE LOW TRANSITION ALT IN USE IN THE UNITED KINGDOM AND THE REST OF EUROPE IS ALSO A PROB FOR US AS WE ARE MORE USED TO THE FL180 LEVEL IN THE UNITED STATES AS TRANSITION ALT. ANOTHER FACTOR IS THE POOR QUALITY OF THE UNITED KINGDOM CTLR EQUIP. MOST XMISSIONS HAVE AN 'ECHO' AND IT'S HARD TO UNDERSTAND THEIR INSTRUCTIONS AT TIMES WITH THE BACKGND NOISE PRESENT.