Have you kept goats before? I am just surprised that you want a billy goat as well as the females. You will need separate accommodation from the word go, fairly strong for the billy, and he will need a companion, such as a castrated male.
If you are just starting out a couple of females, which you could get already in-kid this year (although they would be older than 6 months, probably minimum 18 months just now), should give you at least one female kid in the spring to get a flock (and milking) started - goats herds get established very quickly!
Also you would need a good hay stack handy (or be prepared to buy it in for about 4 quid a small bale) for winter feed. four or five goats go through a huge amount of hay over the winter, and they need quite a bit through the summer as well.