Hi Lopear,
I dont know of anywhere in Somerset or Dorset but know of a couple of places in Cornwall a place called Towan house in Padstow
http://www.towan.comits a self catering 3 bedroomed property with a small secure garden the guy who owns it is really nice and generally doesnt mind how many dogs you bring.
The other place in Cornwall is the White Lodge Hotel in Mawgan Porth not sure if it has a website but certain you could find it if you googled it they used to take any number of dogs too.
There is also a place in Wales in the Wye Valley there are 3 cottages run by a lovely couple who just love dogs and dont mind how many you bring either their website is
http://wyeview.org.ukGood luck
x