between London and Rom you'll find a lot of beautiful places, towns and landscapes. It is difficult to give you an advise. Have you an idea which route you wants to take?
ciao Petra
wer nicht nur eine Sprache lernen will: prolinguis der Sprachenclub in Hamburg
do you already have guidebooks and maps? I think 3 weeks is by far not enough time for this route. It's a MTB route of 2270 km ( Via Francigena in MTB)