Yeah, unfortunately THTC pretty much limits you to SSE. Problem is that THTC uses 2 supplies and most companies aren't setup to cope with that. Did use Coop Energy for a while but honestly couldn't recommend them, they made SSE look competent and fair. You may also find that, if you can get an alternative supplier, they may charge 2 standing charges because there are 2 supplies.
You could ask SSE if they can disconnect 1 supply and put an Eco7 meter on the other (probably chargeable) but you would be responsible for getting your home reconnected to the new meter. This would be more difficult because SSE apparently don't have any engineers in Shetland.
There's also a requirement for companies to work with people on restrictive tariffs like THTC, but this only means that they will offer you their single rate tariff instead and this is not usually an improvement because everything that you use on the heating rate would be charged at normal rate.
Best advice is to look out for fixed rate offers, had a few good ones from SSE over the years, but they are few and far between.