Smyth Superstores have voucher codes for £6 off £50 and £12 of £100. You can print them off and use them in store if you wish, that's what I did, in two different stores last night
On the other hand, Toys R Us are offering £20 voucher if you spend £75 on LEGO, but the voucher has to be redeemed inside some certain dates or something. Check the T&C's on both sites and see what suits you best
eBay is a ****ing goldmine for hard to find stuff. Most stuff still being produced is selling for little or no profit on there and second hand stuff is usually 10+% cheaper, providing it's not a discontinued collectors piece.