I can't help with all your questions but I can tell you that up here (north Cumbria), farmers sell holstein and friesian bull calves at the mart. They fetch £50-£90 -ish at the moment, seemingly being worth rearing as they are large-framed so when grown, even though their backends will be slight compared to a beef crossbreed, there is still quite a bit of meat on there. They fetch a reasonable price for finishing for this reason so there is profit there for people who have facilities to take them from calf to steer.
The calves which fetch very little - I have seen them offered at £5 - are ayrshire bull calves. I haven't seen jersey bull calves at the mart but it is likely they are similar to the ayrshires. My neighbour, an organic dairy farm & cheesemaker, keeps ayrshires and they keep some of the bull calves on to sell as organic beef through their shop and to use in recipes in their organic cafe.
A Jersey dairy farm not far from here processes all their own beef and, like my neighbour, sells it in their shop and uses it in recipes in their cafe. I have had some of their meat and found it tremendous, full of flavour.
Guernseys are larger animals than jerseys, you would get more meat off a guernsey. Although neither of them will be very big at 6 months old. If you are looking at slaughtering so young for veal, fine, but if it is to keep the amount of meat low you may be better looking at someone to take half (or two to take a quarter each) off you. If it is just that you don't want to wait till 2013 then you could look at buying part-grown animals - our local mart sells calves at a few weeks, a few months, at weaning and at pretty much every age thereafter and in between. In fact, it could be a good idea to start with a slightly older pair before buying young calves - they are notoriously difficult to keep healthy when they are very young. Older animals would cope with your playful pony better too (but make sure you get ones without horns!), and then he would get used to calves before you got some younger ones.
On slaughter / butchery costs, it is worth talking to your local butcher (if you have one.) He or she may offer you an all-in slaughter and butchery price, or may be able to do a price per kilo - which could be a better option if you are looking at a smaller animal.
Oh, you have just posted again so I'll pick up on some of the later questions too.
Calves will appreciate access to grass in good weather yes. But they don't like to be wet so you need good shelter. As they don't have mum to take care of them they would probably be best being brought inside if the weather is wet or cold while they are very young. Like lambs, they get cold quickly and you don't want young calves getting wet and cold. Indoor accommodation needs to be clean, dry, very well ventilated.
Cattle are herd animals and any bovine will be utterly miserable without another bovine for company, ideally of a similar age and size.
Bazadaise - yes, we use a Bazadaise on some of our cows. The calves are a tremendous shape and do not need much cake, no. But when people talk about fattening on grass, not needing much cake, etc, they are probably talking about a calf being reared on a suckler cow and getting ma's milk till 8 months old. If you are rearing on milk powder to 3 months you will, I think, need to be feeding cake to 6 months - although someone nearer to you should comment as I expect your grass is quite different to ours!
The other thing about Bazadaise - or any continental breed - is that the temperament may be a little flighty up to downright bolshy. Some are calm and steady but some are not. You would be safer with an Angus or Hereford cross, I think, unless you know a farmer who can vouch for the temperament of his Bazadaise cross calves. Angus or Hereford cross should outwinter ok too. (Don't forget that the mother's breed will also impact the hardiness; if you are looking at a calf from a dairy cow her breed will probably be less hardy.)
I'm sorry I think I have maybe given you more to think about and not very many answers!
BTW, I have that same Valerie Porter book and can recommend it too.