Subsistence and accommodation
When you're working at a temporary workplace (under 24 months), reasonable accommodation and food costs are usually allowable as business expenses. The 24-month rule is the key threshold.
Travel between home and site
Travel from your home to a temporary workplace is generally allowable. Travel from your home to your 'permanent workplace' is not. If you've only got temporary workplaces, you're usually fine.
Keep receipts religiously
VAT receipts, accommodation invoices, fuel receipts, mileage logs. HMRC enquiries can be triggered up to 6 years after the fact. A clean paper trail saves you weeks of stress if it ever happens.