Do you know which grass it was? it could be that the grass is naturally going into winter mode and will be fine. Did you mean ericaceous compost? was this the soil advised? if not then that may be a cause of its change of state. check by looking on the RHS website for cultural instructions for your grass. if it says soil is neutral then it would be better in a John Innes no 3 or multipurpose type compost.

welcome to GoY too.

The days are getting shorter and colder which induces the grass to go into dormancy. When it turns completely brown cut it down to 1/3 of its height and wait for spring. Expect to see new shoots of growth late May of next year. What you should have done is purchased this plant early summer if offered but you came upon it in its end of season blaze of glory and you couldn't resist to bring it home...I've done the same thing many times.

Knowing precisely which grass is critical - 'grasses' covers a wide range of grasslike plants, from sedges to rushes and true grasses. Many are not hardy over winter in the UK either, some are evergreen, many are not. I can't think of one that likes ericaceous soil either, off the top of my head... so if you have a botanical name for the plant you've got, that would be useful to know.

I would have used something like multipurpose compost myself, and in my experience, ornamental grasses in pots are water hogs, even the "drought tolerant" ones.