<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Taxes from Britain which are higher</font>
It's the landing charges that get you, though. A neat little trick to avoid these - LCY is rare in that it's a real 'landing' charge, and not a departure charge. LHR, however, is a departure charge, not a landing charge. So... Flights LCY-XXX-LHR save on the taxes.
The fact that a Y fare LCY-AMS-LCY is nigh on £350 is a different matter, though... But in the Jan T-fares, I was regularly getting LCY-AMS-LHR for £50 all inclusive.