Went to Toys R Us yesterday to buy Lego Boba Fett ship (£74.99 - cheapest i could see anywhere) and noticed the offer instore.
Spend £75 on lego instore and get a gift card for £20 which can be used to buy anything instore between 27th Dec to 17th Jan (from memory)
If you are nudging the £75 mark but dont quite reach it, the cheapest lego item is about £2 (lego mini figures)
I also used my regular £5 off £30 spend from Toys R Us membership card and was still given teh gift card even though teh voucher took it down to below £75. As long as teh pre-voucher price is above £75, you seem to be ok.