歐洲自由行來回機票問題

The cheapest way will be return from London.
But normally, some tickets from HK to a European city can include an internal flight, i.e you can fly from HK to London, London to another city (say, paris, berlin) and then from that city to London and London back to HK.
As long as you plan for the suitable flight, this may be a better choice instead of you buy two single tickets.

the cheapest would be return tickets to London la instead of buying 2 singles!!

Hong Kong- London//Paris,Frankfurt,Berlin or Rome -HKG
return within 4 weeks in oct from HK$ 6300 (all in) with www.klm.com.hk