Remember that agents are in business to make money. If you are vague they are also likely to be vague. Give them something specific to work with and things will almost certainly change.
As for starting somewhere other than London a UK based agent probably can't help you because they have to convert the foreign currency back into GBP if they issue the ticket. Can you do it through the web? Possibly but with a GB postal address attached to your card it might be declined and, in any case, the bank will then convert into GBP to bill you. Mind you the bank will probably be kinder in their conversion than the airline.