I think it is really hard to select cattle that'll give you a return in 6-7 months, particularly if just on very rough grazing. Cornwall, clay - recipe for a baked pan if dry and a heavy cloying quagmire if wet... Not ideal for cattle.
If you were experienced with young calves you could maybe consider buying month-olds, bottle or bucket feeding them for another couple of months, and selling them as weanlings at the end of the summer. But the good month-olds (Angus x, Limousin x, British Blue x out of dairy cows) are fetching £250-£400 at the moment, there's no short term profit there. And you would have to have proper shelter for young calves.