I wouldn't be worried about your sheep, as you say they are pretty resistant and there'll be sources of infection locally anyway.
I wouldn't be worried about visiting bulls nor TB tests either, get a docile breed (eg a Jersey), treat her (and any companions / calves) kindly and you won't have any trouble getting her / them AI'd nor TB tested.
The only worries I would have in your shoes would be about your lovely housecow
getting TB and having to be destroyed. Don't underestimate the bond that forms between a housecow and the person who milks her.

; you would be devastated if she got the disease.
If you are in the South West of England, I suspect you will be on an annual test regime anyway. Defra used to publish a list of all parishes and their testing frequency but I'm not sure they do that now. They do have a page where you can look up your holding (by its County Parish Holding Number) and see what it is.
http://animalhealth.defra.gov.uk/managing-disease/notifiable-disease/bovine-tb/tb-testing-intervals.aspI would do also a bit of research about the TB status of your local dairy / beef farms. If any of them have had a breakdown in the last couple of years, think hard about whether you want to risk your own housecow.
If after all that you decide it looks safe enough and you will go ahead, then
please, for your own and your neighbours' sake, make completely sure you buy in stock from a TB4 area or at the very least that is robustly certified 100% TB free.
And then give some thought as to whether you want to pasteurise all your milk before use, just in case...