✔ 最佳答案
First of all, you have to decide "Where you want to go" !!! That's the most important and the first question you have to solve. Means of transportation is not the major concern at this moment.
Your friends from HK will stay in the UK for 2 weeks, how about the European continent? Will they go there together? How long is your holiday in total, 2 weeks, or even 4 weeks?
Actually, you guys can spend around 3 - 4 days in London city, there is enough sightseeing points to keep you guys staying in London, and then you may consider going to nearby area, such as Greenwich, Windsor, Oxford etc. You can base in London and travel to those cities easily.
Then you can take Eurostar, or flight from London to Paris, which takes only 3 hours and stay for 4 - 5 days. If you still have time to travel, you may consider to Belgium and Holland by train (takes around 4 - 5 days in total) or Italy by train (takes around 9 days in total). We cannot help you to set up a whole plan as we have no information about your planning.
So, please find out "where you want to go" and "how long you can travel" first. After that, come here and ask again, I am sure someone here must help you.