Generally, if you stay away overnight, you can claim for the actual costs of the accommodation and a meal on top of your mileage.  You can also claim Incidental Overnight Expenses (receipts not needed) which are not necessarily business expenses but include items such as toothpaste, telephone calls to family etc. up to £5 per night in the UK and £10 outside the UK.

As an alternative to claiming the actual cost of meals, you can claim benchmark scale rates that depend on the hours you spent away from home (£5 for one meal when away for at least 5 hours; £10 for two meals when away for at least 10 hours; £15 if leave work after 8pm and have to buy an evening meal while travelling home).