Every attraction has Tesco Metro, Sainsbury's Local, or M&S within 2 blocks. Standard meal deal: sandwich + crisps + drink for £3.60-4.00.

Tower Bridge: Tesco at 145 Tower Bridge Road. Westminster Abbey: Sainsbury's Local, 119 Victoria Street. Oxford Circus: M&S at 194-202 Oxford Street. Tourist cafés around these spots charge £5-10 for sandwiches.

Did the math for 6 days of lunches. Saved money versus tourist café prices. That's extra dinners out.