You can reach Austria, Belgium, Czech Republic, Denmark, France, Germany, Hungary, Italy, Liechtenstein, Luxembourg, Monaco, Netherlands, Poland, Slovakia, Slovenia, Spain, Sweden, Switzerland, all on the same day from London.
I haven't calculated the distances but i am guessing the furthest would probably be London (UK) to Budapest (Hungary) as this seems like quite a long distance or possible London (UK) to Malmo (Sweden) may be slightly further.