I'm about to do the SMBLA TCL course (our club is putting a number of people through it for club rides) and one of the requirements is having and using

a bike computer (in km) for use in the navigation section.

Now these days most people seem to be relying on GPS it seems.

What cheapo computer would the STW massive recommend?

cat eye strada

any basic computer your lbs stocks will do fine. All you are looking for is current and trip distances + something easy to use and read. Aldi one is a fav of my roadie mates.
Cateye are my fav, used them for yrs.
http://www.cateye.com/en/product_detail/254
Enduro 8 is what I would buy. Have not much faith in cordless reliability esp in wet/cold weather.

GPS = Garmin eTrex imo, £100ish or cheaper from amazon

I got a catseye, one of the wireless ones because they are much tidier. I didn't have much problems with it other than falling off and loosing the transmitter! For the GPS, I'd second the eTrex. They have the same functionality as the expensive bike ones but are a lot cheaper, robust and easy to use.

It's worth saying that nobody should be relying on a GPS. Signals come and go, batteries die, etc etc.