Oh sorry, should have made that clear. I contacted the consular section of the embassy. They contacted the Foreign Office in the UK who basically told them that they won't be able to provide this letter. The embassy advised me to contact a UK Notary Public and ask them, but that will be bloody expensive.
The notary fees are out of control here...
I could call the other two consulates (one in Dusseldorf and the other one in Munich) but I doubt their replies will be any different.
Oh well, I'll contact the FCO myself. Might get lucky.