Prices are not that bad for tyres, do you have 15" or 16" wheels ? (I'm guessing 15" as 16" tyres are pretty pricey and it is a rare size).
Are you able to fits brake bits yourself ?
You can get rear discs for less than £40 each from Eurocarparts or similar, and rear pads as well very cheap (rear brakes don't do much anyway). I would go for better quality on front pads (and discs if they need replacing) as they do up to 80% of braking - but you can still get good prices. On some cars specially if your alloy wheels are very 'open' (big spaces between spokes) a lot of crap can get on the discs and calipers and cause rusting discs and problems, steel wheels do protect the brake parts a lot better.
As far as driving on tyres with 2mm of tread, I would not do it as on wet roads they will aquaplane easily.
Brake pads are an easier call as they will work as long as there is material present, but should not really be used below 1.6mm (you may get a loud squealing around this figure as the 'alarm strip' does its job and rubs on the disc, as material gets thinner there is a danger of pad steel backing rubbing on discs and tearing up the surface, this will definitely mean new pads and discs straight away).