As Phil states BV have a 3 year contract, so there will certainly be an SGP in Manchester next season, as there is no way the GP Promotors are buying themselves out of that deal, and why would they want to after 2 successful years.
The only caveat I will put on that, is if the 3 year deal was contracted for 1 SGP only, and this year they have only offered BV 1 SGP and we can't make that Pay, we will know soon enough on that one.
As for the long term it makes sense, if possible to have more tracks in each country that can hold a GP, Tracks in Poland come and go and come back again on the SGP calendar so why not the UK. If you have Glasgow, Manchester and Ipswich as your 3 tracks, and say we get 2 SGPs per year then you could share that between 2 tracks each year with one track missing out every 3rd year. That would add the variety and keep it spread across the UK to enable more fans to go. I think Glasgow would be good to go (not sure on the Capacity though), but the Ipswich track would need widening, its just to small for a GP IMO and would not give us the top class racing we want to see. Not saying that cant be done, but it certainly needs doing.