A great recent discovery has been Groupon - you sign up and then receive an email every day letting you know what discounts are on offer (there are usually three+). You sign up by UK city so the deals are all local - they are mostly beauty or health-related treatments but they do have restaurant, theatre and holiday deals too.
If you are in London, you get double the choice with two emails a day.
I have already booked several deals through them - beauty treatments which have been up to 70% off - and I have booked a spa day at a top London hotel with some girlfriends which was 80% off!!
When you buy, you are emailed a voucher which you then have at least three months to use - you phone the salon/restaurant/company yourself to book what suits you.
Take a look here!!