Hi Schvuntz,
Sounds like you're taking the same road I took. Shocked at the price of firebrick in the UK, I used a fireclay & grog mix instead over a sand form, then dried it out with a halogen heater. I'm still finishing it off, but am getting ever closer to firing the thing, so expect more posts... My oven's been very cheap (thanks to lots of recycling, freecycle and ebay), so it can be done!
I've got a lot of the details on this thread:
My oven build posts (Another UK oven started)
There's a lot more detail on my blog - see the link at the bottom of this message.
Couple of UK-centric tips that might be of use: the cheapest clay I could buy was fireclay (handy), try builders merchants as it's more expensive from pottery suppliers. The cheapest vermiculite I could find was from Travis Perkins, at £7 per 100 litre sack.
Carl