Originally Posted by FrancisA

This is quite different. You can only MFU a ticket issued in the country of your BAEC membership. If you book a revenue ticket ex-EU, it will price up (in euros) and ticket in the EU country where the trip begins. If your BAEC membership is not in that country then in theory you cannot subsequently MFU the ticket. [I say in theory as you can get agents who do not know or ignore this rule.]

If you use "book and upgrade with Avios", it will use the cheapest fare for the route available for sale in your membership country. Thus if you try LIN-JFK, for a WT+ to CW MFU, the euro priced ticket is likely to be much cheaper than the cheapest LIN-JFK ticket offered for sale in the UK.

I don't think it is a case of finding an experienced agent, more find one who does not know or follow the ticketing rules.