Perhaps ask Mab ( he sometimes comes on to the forum ) , he has a bit of history in this sort of area & is not that far from where you might be looking .
possibly my case is similar - I bought a place (30acres of slope a goat could fall off) with a 'chalet' - actually started life as aa double static caravan, then had a conservatory built onto it - so legally no longer a static but a chalet. The important thing is it's been lived in for over 40 years, council tax been paid on it, services connected, etc (i've been here over 4 years now), and it has residential status so I should be able to get replacement dwelling permission (I've had the planning officer around and he thinks there shouldn't be a problem) - I'm actually in the process of submitting an application (keep your
please folks!). Also I'm not in any sort of conservation area.
if 'your' house has been a full time residence for many years with council tax paid then you should be in a good position - although I'm no expert.
I'm not going to be able to offer much advice on what to do with the 20acres - if it needs to make money for you then good luck but it won't be easy. If you have a day job then you can do smallholding for whatever you want (assuming no local restriction as others have said).
It's worth doing the research though - have a look around, check OS map for public rights of way passing through, etc.
I'll be interested to see what it actually goes for - if the existing house is good I'd expect it to go for quite a bit more. I paid ~150 for mine (not auction), but my house wasn't 'good' - in fact the estate agents particulars described it as "could be habitable... with a bit of work..."
a very optimistic description THB.