[Edited]I am a big fan of
[Edited]
I am a big fan of 3/4 tights (stops just below the knee). They work great down to about 8-9 degr. Celsius. After that, I put add a pair of 3/4 longjohns, which gets me down to 4 degrees. After that, I get out my long tights. At about 0, I put on longjohns underneath. All of this assumes similar layering with the upper body clothing, which is absolutely essential if you are going to go fully barefoot through the winter. A wool hat is also essential.
I have 3 pairs: aisics, north face, odlo. All good, but the odlos are losing it in a seem. I can usually find them discounted at outlets or on sale (not in the Fall, of course). I have paid between 20-40 Euro (all discounted). [Edit]: I tend to avoid cheap no-names, because I want quality--since they last longer and is cheaper in the long run. But, note a brandname is not a guarantee of quality. I actually hate having logos everywhere, and will remove them where possible.
My long tights are odlo, and they are great. They usually come in two different weights. Mine are heavier, i.e. a fuzzy, thin fleace inside.
I am very anti-cotton when it comes to sports gear. If it gets wet, as I trust is an issue for you in the UK, you'll be miserable. All of the above, including the longjohns, are various sorts of polypropelene, capalene, polyester, nylon--whatever the heck all that is. It all works quite well, whether the wetness comes from outside or from in.
One important thing: when you wash them in the machine, remove them immediately. These fancy nylon-based fibers start to get a musty smell if they are left in the machine too long. Observe the drying instructions.
Best of luck
Paleo