Firstly, please be aware that throwing the final sector is not really something to promote. It would be far better to say that you pay the 200 Euro for the stopover in London, as an option on some of these tickets, and then you can use the last sector to go and pick up another ticket.

Also factor in the cost of going to pick up the ticket. If you take the ATH off LGW on easyJet and book a four hour turnaround, then this will get you into LHR late enough to miss most of the flights (depending on the route) and then you can nightstop London (if you live around there), pick up your bags and then get on with the main trip.

This option may not autoprice in some computer systems but can be entered manually.

You can now buy these tickets in the UK. The fare differences are not using LON prices, however, I am not sure what the differences in the prices are. Remember that you must have the final prices to compare as some of the taxes can be horrendous.