Hot wheels at Toys R Us deal
Buy one get one half price
They already have some reductions on sets, so makes it even more of a bargain.
And, if you're not looking for big sets but stocking fillers, they have a range of £4.99 track pieces like loops etc., so 2 for £7.49 and single track pieces are only 99p each, so 2 for £1.50. I can't find the track pieces cheaper anywhere else than this deal.
Also note...they've reduced their track pack from £29.99 to £19.99, which has great reviews. You can therefore get 2 for £29.99 - I saw a second hand nearly new version go for £19 (inc postage) last week on ebay.