If you are visiting London from outside of the EU, then you can save loads of money with tax-free shopping. Almost any item you buy in the UK, you will be paying VAT on. However, you can claim this back. That means you can get a full 20% of almost anything that costs you over £30. That means 20% off clothing. 20% off electronics. Almost anything!
Through the link below you can find out exactly how you do this, as well as finding some great shopping guides to London.