I have done a similar thing.
Where we live in North Yorkshire the village has a very poor broadband setup (the cabinet is ~2 miles away in Helmsley) so we managed to get a wireless solution and now enjoy a 12mbit connection.
We have a Gigaset N300A base station and four Gigaset handsets paired to it, there is also a DECT repeater in the house as we don't get full coverage around the house (thick walls).
We have two SIPGate basic accounts and have set two handsets to one number and two handsets to the other (though you can mix and match as you like with the N300A).
The service works REALLY well and we can make multiple calls in and out simultaneously, call eachothers handsets etc..Its a total shift away from the 'normal' way of using a landline phone, people can still get through to the other handsets when you are on a call, you can setup times the phones go straight to voicemail etc...
For outbuildings we could always get another N300A to cover that if we wanted, no need at the moment. For an office you could use a corded ethernet based phone which just contacts the SIP servers in the same way as an N300A would (I think they should coexist on the same account without issue!?)
Who you choose as your provider is up to you but I've used SIPGate for years on and off without issue.
Whatever you use I would try and see if you can get away with not needing a server on-site, Voipfone and SIPGate have some decent deals for a small business im sure? Its just one less thing to be troubleshooting.
Good luck, good choice ditching BT aswell
Moorsweb rural wireless broadband
12Mbit down / 7mbit up, great service.