Although MyTravel and DHL may operate the oldest B757s in the UK in terms of years, if you look at hours flown there are some much younger B757s in the UK that have flown more hours.
Compare these 2 ex-British Airways B757s now with DHL...
G-BIKC (built 1982) had flown 29558 hours in February 2000
G-BIKF (built 1982) had flown 38001 hours in September 2000
...With these younger Air 2000 B757s which have flown with the airline since new
G-OOOB (built 1987) had flown 60194 hours in April 2001
G-OOOC (built 1988) had flown 61980 hours in April 2002
I think these figures demonstrate how much harder charter airlines work their aircraft than their scheduled counterprarts - the Air 2000 aircraft have flown roughly double the number of hours as their British Airways counterparts even though they are 5-6 years younger. The flip side of the coin is that the scheduled aircraft will have a high number of cycles (around 1 cycle per flight hour) while the charter aircraft operate longer stage lengths (around 0.3 cycles per flight hour) due to their average stage lenth being slightly over 3 hours.
Air 2000s oldest B757 G-OOOM (built 1986) had only flown 34237 hours in December 2002, betraying the fact that it spent it's early career with Eastern Airlines in the US before joining Air 2000 in 1990.
Monarch's oldest B757s also have a high number of hours, but don't appear to have been worked as hard as their Air 2000 counterparts...
G-MONB (built 1983) had flown 59805 hours in February 2000
G-MONC (built 1983) had flown 63137 hours in April 2002
If the figures are extrapilated it would support Steady Eddie's assertion that G-MONB is the highest time/cycles B757 in service. With G-OOOB and G-OOOC about to be retired (G-OOOA has already gone) they probably won't catch up...
While MyTravel do have some very early B757s, they again flew with Eastern Airlines, and then spent several years in storage before joining Airtours in 1995 so these aircraft aren't actually that old in terms of airframe hours...
G-MCEA (built 1983) had 48092 hours in March 2001
G-PIDS (built 1982) had 48882 hours in February 2001
G-RJER (built 1982) had 49351 hours in February 2001
All the figures are available on the CAA G-INFO website:
Hope I didn't bore you with numbers too much! Just wanted to show how that how an aircraft is used is far more important to its "age" than how long it has been in service