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?
First of all it is in your control. Everybody has different strategies in this market. If I were you, I wouldn't sell half of my BTC but a little of them would be sensible to enjoy spending on something.

When it comes to altcoins, it is really hard to make a decision among them as there is a bigger risk there. You should analyze them very carefully to make a good choice.