In the UK, how can i work out how much my retail business is worth for
resale purposes?

The best way is to consult a professional valuer.

In any event, when you do put your business up for sale, the agent may
suggest this.

Of course, it costs money and the chances are that a purchaser will
also need a valuation to satisfy his lenders but the purchaser cannot
rely on yours nor should you on his.

It very much depends on the type of business that you are in; its
future prospects; the value of any property; and cash flow
considerations.

Good luck!