Hello Sam,
I am not clued up on all planning laws , but I think that even for a farm building you now need permision. To become a smallholding or to be more correct ...to get a holding number , it is just a matter of applying for one from the right department . The link here should tell you all you need to know:
http://www.smallholder.co.uk/mostpopular.var.927599.mostviewed.registering_a_holding.php Without a holding number you won't , by law , be able to keep livestock on the land ,so you really will need a holding number! As far as disadvantages , well they know where you are once registered....lol, but without registration you wouldn't be able to function (legally) as a holding !!! Once you have your number , you will be able to get any livestock you want , along with all the paperwork . I don't know if there are any real advantages as such , just that without a number ...no animals....
The vegetable patch ....well , there aint nothing like good old hard work ....out with the spade and fork , who needs a multi gym? The fruit trees maybe just dormant still , all depends on the climate where you are . I think you are a bit ? colder than round where I am ( west Wales) so it may be a few weeks till they spring into life . Hope that helps a bit......
cheers
Russ