Most museums and galleries in the UK, including the likes of the Natural History and British Museum in London, are not just free but are also among the finest museums in the world. The same can be said for the National Gallery and the Tate Modern.
Ok., so this one is not limited to London, but just like a lot of cities there are some great happy hours and night time promotions in London's clubs and bars. Chains like Late Night London offer some great deals in the first few hours after work, including two-for-one drinks and food.
There are plenty of chains in London that do some fantastic deals with vouchers that you can download to your phone, or print out, for free. As with happy-hour hunting, it's worth having a look on voucher websites and those of the individual restaurants ahead of dining; just remember to let the waiter or waitress know before you order if you're using a promotion.
London has many fantastic markets and they can be great places to buy food, clothing, souvenirs and pretty much anything else. They can be a lot cheaper than department stores and are always much more enjoyable places to spend time.
All of the supermarkets in the UK are constantly running offers on everything from fruit to Champagne. When shopping, make sure to look out for these (usually marked with a red price tag). If you're looking to buy more than just food for a couple of people, it's worth doing some online research first as you can regularly save around 50% on the usual retail price.
It can often be just as quick to walk between Tube stations when you're in Central London. A decent rule is that within the Circle Line, anything 3 stops or under is walkable in about 20 minutes; this not only saves time, but also money on the underground ticket.
Visit Sam Smiths' pubs for the cheapest pints in London. The pubs are all over London and the Taddy lager is usually just over ₤ 3 a pint, compared with an average of over ₤ 4 for a decent beer in most London pubs.
Go up the Walkie Talkie building, at 20 Fenchurch Street in the City (London's financial district), for views over the capital city, instead of The Shard or London Eye. Entry to the Sky Garden is completely free, although you do need to book.
Look for alternative airport transport from the special trains advertised. Buses and local trains can be much cheaper.
Drink coffee and buy sandwiches from independent shops, not the large chains. The Speakeasy Coffee & Brew bar is a perfect example and in a great location for relaxing after Oxford Street shopping.