as has been mentioned, where is the new fibre cabinet to your house. I had a 5M service with sky and transferred to fibre and managed to get the 40M service I paid for. The green cabinet was 2mins walk from my house.
I would say if you can get 15M+ and you are willing to pay the £20/mth then go for it, if not then wait until a neighbour gets fibre and see what speed they get. A 15M service will run sky download and 2-3 people downloading at the same time. It depends on what the overall use is in your house, if you notice any difference or not.