If you are UK based like OP this is generally true though ex-TLV has lower taxes than even ex-UK or did a couple of year back. If you are US based two one ways are basically always going to be cheaper. Right now the surcharges/taxes ex-US roundtrip to London in Upper are $2300 which is often more than 50% the cash price. I have actually never flown VS ex-US for this reason.