I'd say you'll max out on hrs with FR anyway (900 p/a), and if you didn't you'd need their permission and conform to CAP 371.
If you decide (like a fair few I know) to instruct on the side without telling anyone then this is all fine until something happens, and if they go through your flying experiences with a fine tooth comb they'll throw the book at you, so in short, if you don't get their permission don't do it.
D777