Grain Connect fibre broadband

The house I'm moving into has "FTTP fibre to the premises" which I have never used before. There is already a Grain Connect router in place and I can take a 7 day trial without any obligation to buy. They charge £25.99 per month for 100MBps and £32.99/300, £39.99/500 and Top speed is £49.99/900MB. That seems competitive compared to what I paid plusnet and shell before.

Does anyone have any experience of using Grain please? Good, bad, indifferent?
Can I choose any fibre provider or am I stuck with Grain?
Thanks