If you are looking for something FREE to do in London you might want to consider attending a free concert. Two places that offer free concerts are York Minster which holds concerts on most days. Evensong begins around 5pm with a chorus and this is one time you can visit the church without having to pay a fee, although donations are always appreciated.
Next, we have St Martin-in-the-Fields church. Located in Trafalgar Square in London, they offer free lunchtime concerts which are performed throughout the year Monday, Tuesday and Friday and start at 1pm. The concerts are classical instrumentals or chorals. Seating is first come, first served and a donation of $5 is suggested. To reach the church exit the Underground at Charing Cross station on the Northern or Bakerloo line and walk northeast. For for concert information visit stmartin-in-the-field.org