On slaughtering in the UK you have two choices - slapmark which farmer describes in detail, or a metal eartag (metal as it needs to survive the scalding process which pigs go through after the kill to remove the hair).
Slapmark will cost you around £86
http://www.fearing.co.uk/product.cns?folder/=&cID=14&element129=3&pID=562but will last a lifetime.
Eartag pliers will cost around £26
http://www.fearing.co.uk/product.cns?pID=577&cID=14with tags around 56p
http://www.fearing.co.uk/product.cns?folder/=&cID=14&element129=2&pID=576So you can do off 100 pigs before the slapmarker works out cheaper (at 6 a year that's a long time!)
Both have your herd number on eg AB1234
With slapmarker, you will either see both halves marked, or if the abattoir cuts for you the shoulder joints should have the mark on them. With an eartag, if you take halves, then one half will have the head on with the tag and the other will be from neck down. You can however clearly see that they match. If the abattoir cuts, then you will not know. Now abattoirs do care about you getting your own carcass or meat back, and whilst mistakes can happen, this should be very rare, and we did not worry about this when we used eartags in our early days.
As for worming, if you got them wormed (and you should ask the breeder when you buy) the you would typically worm every 6 months, so if taking them off at 6-8 months they would be due when they are ready to go, so no point. If they were not wormed or if you keep longer would be worth worming, but a Farmer says check for the withdrawal period (the time between giving it and when they can be slaughtered - the packet will state this).
As for resting your ground - if it did not have livestock and your weaners were wormed before you got, and you don't have other livestock very near, with 3 weaners I would not be worried about resting for worms, but would be good to maybe use two halves to allow one half to regrow - as Farmer says it (the ground) will benefit from rest.