So I bought BTC at £9000, made a nice amount. Is it a good idea to now sell this and move that money into alt coins? or should I sell half BTC and buy back in after alt season is finished?
What do you think?
You are asking us? All answers you will get will be to keep holding. What was your plan when you bought? Since you obviously entered Bitcoin to make profit if you are asking when to sell. Without having a clear plan and targets set then you can consider yourself lucky to have made this kind of profit.
My plan was to hold, which I did, I may take some profit and use that for Alt season. I don't know about lucky - if you buy and hold then sell at the right time you make money.