The 24-month rule dictates where your normal place of work is. If you have been contracting at the same client location for 24 months then that location is deemed to be your normal place of work and you will no longer be able to claim expenses for meals at this location or for travel to this location.
This also applies if you have multiple clients in the same vicinity. For example if you have been working at Canary Wharf but for multiple clients over 24 months the rule will still apply.
The 24-month rule is forward looking. This means it applies as soon as it is expected that you will be at the same location for 24 months, whether firmly contracted or reasonably expected. For example if you have been working at a location for 18 months though a change to a project means that it will take a further 7 months to complete then you should cease making expense claims for meals and travel costs immediately.