Mine is with SEIB, I can't tell you the cost off the top of my head as it was a bit ago that I sorted it. I went with them because they offered a breakdown service too.
After a lot of shopping around I bought my insurance from SEIB too. I already have my 2 horses with them.
I did it online which was pretty straightforward and I think I paid around £60. When I googled "Horse Trailer Insurance" they came up first in the list so figured I would keep everything together in the end!
Always had really good service from them so definitely worth a look!
My Equitrek is insured by Anthony Evans. £140 per year. (They are a little bit more than Ifors, because of the higher purchase price, therefore, I thought this was very reasonable)