Taxpayers address - IT Contractor

I have an IT contractor client whose primary residence is at his house outside of London but he rents a flat in London purely for business purposes and claims the expenses through his Limited Company. As he spends his working week at the London flat, is it okay to change his personal address for SA purposes with HMRC to the London one without causing any potential problem or query from HMRC?

The same applies for the company in respect of VAT and PAYE, the address registered with HMRC is his home address but would be far easier if it was London.

I don't want HMRC turning round and saying that London is now his permanent base solely because of that.

On a related note, he spends the working 5 day week in London and the weekend at home. It's been suggested that he should only claim 5/7ths of the London flat costs, however I intend to argue that as the company can't sign a 5 day week lease with the landlord the full 7 day cost should be a company expense, regardless of whether the client stays on in London at the weekend or not. He still after all has his computer and office space set up in London whether its sitting idle or not.

Any thoughts on either issue would be appreciated.