A lot will depend on any SOR arrangements you negotiate with your suppliers, how independent you want to be or if you are looking at a franchise arrangement.
Location will play a big factor in costs - running a shop in a small section of a garden centre will be less than renting a shop in the centre of a town. In a run down town center such as Stevenage there's a shop up for rent at £19,000 pa. in the town centre, and another in one of the outskirt centers for £12,000 pa. If you look at a more affluent area such as St Albans (which is also a little nearer to London ) a reasonable size shop on the high street will set you back £97Kpa in rent
Then there's rates, electric etc and fitting out costs - My guestimate would be anywhere between £40,000 and £150,000 outlay for the first year of trading, depending where you base the business, how lavish you want to fit the shop out and what arrangements you get with suppliers. What income you get will also be dependent on location, profit margins etc. Adam from TA exotics has not long taken the plunge and set up around 6 months ago. - Might be worth asking him for some ball park figures https://taexotics.co.uk/ He might not want to divulge too much info, but could give you some idea of which end of the spectrum you might be looking at.