You can claim actual meal costs whilst you are working at a remote site i.e. away from your normal place of work or when staying away from home overnight however, daily claims for meals are not permitted. It is the Inland Revenue's belief that you need to eat regardless of whether or not you are at work (you need to eat to live not eat to work) so the meal costs are not wholly attributable to the contract and can therefore, not be claimed as an expense.
A subsistence payment is, by definition, one that does not have to be supported by receipts. The only subsistence payment permitted by the Inland Revenue is £5 per night (£10 if abroad) for each night spent away from home during the course of the contract. You must be able to produce receipts for any other costs that you claim as an expense; the Inland Revenue requires that you keep your receipts for a period of 5 years.