When choosing a breed for really good pasture such as I guess you have in Kent, I would urge you to avoid a northern / hardy type such as Highland, Galloway, Shetland, Blue Grey, Red Devon. All of these breeds are likely to become obese on your good ground!
Rearing calves on a bucket does require some experience and/or some luck, but if you do decide to go that route then get ones who are at least one month old and are in tip-top, glowing health, to give yourself a fighting chance
A cross calf - beef bull on dairy cow - would be the sort you'd be likely to be able to buy at a young age.
A weaned suckler calf, reared on its beef suckler mum, sold at 6-9 months old, would probably be very standoffish /flighty at first but as has been said, a bit of cake will help to settle them. I'd avoid continental breeds and choose Angus or Beef Shorthorn, or your local breed if there is one. Definitely avoid Limousin and Charollais. British Blue might do too well on your ground and have heart problems.
I have friends with Dexters who absolutely love them, and I have to say theirs are very nice-looking cattle. Some Dexters are not as quiet as others, so make sure you get ones from calm stock
Jerseys are a small breed but only useful for milk and breeding really. Once you are experienced, then a Jersey cow will give you milk for the house, a calf every year, and rear a few bought-in calves too.