Some general builders will install it and just get a Part P electrician to certify it. Some will just do it and not bother worrying about the Part P stuff--just like they use to do a few years back. £725 sounds well over the odds.
Another option if its possible is to have the bathroom fan ceiling mounted and ducted to the eaves through the loft.
What I would do is write down what you want and find half a dozen sparks who advertise and have emails. email it off for quotes or at least a budget subject to survey. dont mention how much you have been quoted,or let them see who you have emailed to.Do you want them to supply the fans --in which case specify what fan you want and how operates pull cord, off the light switch or based on the rooms humidity. Might need to know room size to determine fan size. try www.yell.com