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.
Utter codswallop.
You most certainly
CAN buy tickets non-UK start tickets from a UK based agent.
The only time you buy tickets that don't originate from your home-turf is if the savings are substantial, in which case the risk of agent passing on a couple of percent in forex costs is not likely to make a big difference to your life.
I seem to have a vague recollection that IATA set currency rates for tickets anyway ? Neutral unit of currency or something ?