What to do with £600 any suggestions please?

You have a well-rounded set-up as it is; there is no point in spending a ton of money just for spending's sake.  You'll just end up with a bunch of expensive equipment you'll never use.

Use the equipment you have. Work with it until you identify something your equipment is lacking.  If you find that you enjoy landscape photography, and want something wider than 18mm; or if you like close-ups and want a macro lens; or you discover portraiture and want a flash; ​that​ is when you want to go buy new equipment.

If you're saying to yourself, "I have $600; what should I buy?", then the answer is "nothing", because its clear you don't need it.  When you're saying to yourself, "I have $600 and I know what I want to buy", that is the time to spend the money.  Until then, keep it in the bank.