When I was choosing I spoke to a nutritionist and now feed a cup of balancer a day instead to make sure they had all nutritients. The improvement in my veteran has been completely astounding.
It doesn't disintergrate as quickly as others .....I use it to try and stop mine ever getting Equine Grass Sickness and just generally to make sure they are getting all the minerals they should be.
I have used "Uni block" from Dobson and Horrell. It is molasses based but it is not soft like the other brands and does last very well. 4 youngsters have had one all summer and there is 1/4 left.
I get mine from Countrywide. Rockies are the biggest makers of weather-hardy mineral blocks. I get the five star and use one laminshield block (hi-mag) in the spring. There are other hi-mag blocks available but I personally prefer this one. 5kg blocks from rockies.
10kg blocks are available but I haven't seen anyone stock it in the UK.