£5 sounds as good as it will get.
What is megabus policy, as they run twice daily Penzance to London.
Megabus' policy is as follows:
"megabus.com does not check luggage or provide receipts for luggage transported on the bus by the passenger.
We don’t mind what your luggage is packed in, we only ask that it weighs no more than 20kg and be no bigger than a standard ‘large’ suitcase. The maximum size of a single bag is 200cm (width + height + depth).
Alternatively you may take two or three bags so long as they do not occupy more space than a single bag of these dimensions or exceed the maximum weight limit of 20kg once the luggage is combined.
You can also take a piece of small hand luggage on board with you and it must either fit on your lap or under the seat in front of you. Please be advised that passengers wishing to travel with more luggage than one standard sized suitcase and one small piece of luggage that can fit on your lap or under the seat in front of you, you may have items refused."
Personally I always like to try and get the train wherever realistically possible
As an aside I don't imagine the Thames being that good for surfing.

I'll let you know on that one!
Km