Load up matrix.itasoftware.com.
Complete the "From" section as AMS;ARN;ATH;BRU;BUD;CAI;CPH;FRA;HEL;IST;LIN;LIS;MAD;MLA;OSL;TIP;VIE;WAW :: BA+. What this does is select all the possible starting location in Europe
Put in "To" as YYY :: BA+ (YYY is your destination).
Select some rough dates and LON as "sales city" (so you get prices in GBP).
Click more options, change cabin to Business or First and uncheck "Check seat availability".
Click 'Go'. Click 'show all options' and scroll down if you don't fancy the first options (TIP/CAI/IST) or just delete them in the first place. Sometimes you need to click on 'Cost' before they will list in order of price.
Note that all advice is offered solely for information, and is just at my judgement. No legal liability is accepted if you take my advice.