Assuming Sundance passes the MOT tomorrow

, this year's main plan is to get the underside checked out, tidied up and treated.
I'd already done some prep work on the sills (which seem to be in really good nick), but it looks like I will have some cash spare to get a professional job done on the sills/underneath. Which I think is preferable to me doing it DIY, and anyway I no longer have anywhere to do it myself.
What are my options for doing this? I'm not sure about Waxoyl as I gather it has to be done every year or so (is that right?) Preferably I'd rather get the underside cleaned up and then permanently coated in some way.
If that's an option, where can I get this done? Am in London but will travel.
Cheers
S