What's the basis of people saying there is a problem with running a business from home? Lots of people have home offices, a lot of small businesses (web design, curtain making, eBay based sales) are run from a residential property.
My understanding is that so long as you've not made alterations that require planning permission, you've told your mortgage company, and sorted out insurance you're pretty much OK. So long as you're not using parts of the building exclusively for business – eg your office also gets used as a spare bedroom or a second TV room – business rates/CGT position doesn't change.
Running a B&B, which you've permission to do, you need to provide secure bike storage and parking. Making a workshop available to guests is no different to letting them use the TV room as far as I can see.
The only issues I can see are around noise and disturbance to neighbours and you have to be making quite a bit, and usually at unsociable hours, before anyone has any valid grounds to complain (and even then it's a slow difficult process and they'd need to provide a lot of evidence).
Even if you *were* putting up 3 pairs of mtb'ers and their bikes every night, repairing their bikes in your back garden in the early evening, and setting off with them in a van you owned to go to the trails each morning at 8am I'm not sure she would have any valid grounds to complain.