£20 cash back when you get an Amex Platinum Credit Card and spend £1 - Excellent card with 1% cash back on all spending. More wildly accepted now - and has some great offers.
£50 Amazon Voucher when you sign up to BT Broadband. If you are struggling with broadband quality switching to BT is often the bets way to get it resolved - you can always switch away again after initial period. Great for BT Sports & Sky Sports as well.