No, but rather than phone a "window" company, to keep the cost down it might be worth finding your local Rehau trade supplier and asking them for a supply-only quote. My local one is
http://lostockhallglass.co.uk/ but they're a bit far from Dorset....
Either fit yourself (which isn't that difficult), or find a builder/handyman capable of fitting windows (if it's a bay, is the existing bay window structural - many timber ones are), or are the bay posts brick/stone?). If the fitter isn't FENSA/CERTAS technically you should do a building regs application if you don't use an approved fitter, but with the units on supply-only you'll save the window company's overhead and profit on the units so the ~£250 BR fee might be worth it... - Depends on your attitude to warranty - which you obviously get if you have a window company do the whole job.
If you have existing sash windows you'll need to pay attention to the detail of how you work with the existing reveals currently housing the sash boxes. You could fit the new windows into the outer skin (which IMO can look odd) and then just build out the reveals, or you could re-work the reveals so that the new windows sit where the old ones do - just slightly behind the outer skin as the existing box sashes are likely to be, so they "look" right.
If doing this pay attention to the lower corners - the old box sash cills will be shaped so water runs out over the (stone/brick/tile) cill, which is likely shaped to deflect water outwards. If changing to uPVC you don't want any water running round the existing cill into the wall structure. Even if there is e.g. a stone cill, you'll most likely need to specify new uPVC windows with a cill (which come in different sizes) rather than without., and take care with the detailing of the lower corners.
When I have blocked in a box sash reveal to take a standard window, I've used thermalite blocks (easy to cut) and mortar (remember to fit a damp proof membrane where they touch the outer skin bricks) then fitted the window and then finished the inside. If they are sash windows they may well have architraves around the internal window opening, so you might have to plan to re-work these too.
Either way, replacing box sashes with uPVC you are almost certainly going to have to do some associated builder/joinery work, and window fitters may not have the inclination to do the best detailed job (they'll do the fastest job).
Apol if I'm teaching a grandmother to............