As I stated in my original post, what you could is start out with trial offers (ie. discounted offers). Perhaps you sign up online and the first 5 meals carry a 50% discount...or similar.
Once people are hooked on what you are offering (providing what you are offering is of high quality), they are more likely to part with their cash, once you hike up the prices.
This idea definitely has legs to run, however, initially you want to offer this service in a city centre (preferably London, Bank area). You may also want to look for affiliation with one of the big gyms in Central London - Holmes Place, Barbican springs to mind, as that gym has a lot of wealthy clients.
Personally, I think the idea will either fail/succeed on the strength of the quality/taste of the food you offer.
You may also think about a service where you deliver the food directly to the people in offices (this will have security issues so the delivery will have to be left with reception).
If you do ever launch this idea, by all means post the link in this thread as it will be interesting to see it develop.