Yes is the simple answer despite what some tax inspectors think.
The estimates need to be reasonable, have a logic to them and be an expense that could not easily be calculated any other way. Also you would not expect to estimate most of your expenditure.
The best way to successfully make a tax claim is to have a proper receipt or invoice but for some things it’s perfectly acceptable to estimate.
If you’re having trouble with HMRC over estimated expenditure then maybe we can help.