There are actually only three formulations in the UK. (Well, for a while yet, there is the ever-so-slightly different formulation of 50mcg Mercury Pharma levothyroxine - see link below.)
The establishment often appears to assume that they are interchangeable. Some people seem to manage to switch with little bad effect. Overall, though, we see a lot of people finding one much better (or much worse) than the other two.
I don't hold with reputation - it is what works for you that matters, not what others might have thought.
There have been quite a few papers that have shown switching can be a problem. Much of that research was done in the USA where there are considerably more formulations available (though fewer than even a small number of years ago).
Although I have tried to pretend that they are all the same to me, I have reluctantly come to believe that they are not. On Actavis I feel overall under-dosed. On Mercury Pharma I feel over-dosed shortly after taking them. I have not had a prescription of Wockhardt-only since I was starting.
A classic example of them not being the same was the Teva formulation which had its product license withdrawn (for the 100mcg dosage). See the report linked here: