從London坐巴士到Paris

1. are you coming to London via Oasis airline? that could be the reason you think you may delay.
2. I think you should keep your option open, you have the following choices:

Euro Star, if you book in adv the cost is low.
Air, some if you could book in adv the cos is very reasonable, and you can take off from Gatwick, so you don't have to pay extra to go into town, because either option you still have to travel into London. Check the expedia website, which give you some ideas of prices. Even you think your flight may be delay, you can ask the airline if you miss your trasfer flight can they put you to the next flight and what's the cost. This will be the fastest way to Paris.
Bus, you can check have they got bus from Gatwick.

I have to say the cheap flight from HK sound very good but Gatwick airport don't have a good link to the city, which may cost you more at the end.

http://www.expedia.co.uk
http://www.nationalexpress.com/eurolines/home/hp.cfm
http://www.eurostar.com

anyway those are the website you could get some prices, you can book your ticket directly with the airline.

have a good trip, don't forget Europe is using Euro and UK is £......

From Gatwick, you can take Eurostar to Victoria Coach Station in London and then transfer to another bus going to Paris. Total cost is 52 pounds one-way and the whole trip will take about 13 hours (you can take an overnight bus and arrive to Paris in the morning).