While I could never suggest a move to a larger, more rigid and more powerful milling machine would not enhance anyone's workshop I don't think you will get it within your budget buying new. ArcEuro have discontinued the Sieg SX1L mill which is basically the same as your present machine but with a longer table and travel. You could add these to your present machine I suspect for less than £100. The replacement in the ArcEuro range has a more powerful motor and stronger drive but is not much larger. Going any bigger takes you out of your price bracket, particularly when the larger tooling necessary is taken into account.
I think you have two realistic options – Firstly look at the secondhand market, but here I cannot comment as I bought the Sieg SX1L new as it was the only machine that I could fit in at home and anyway I had access to a larger machine at the club (then!)
Or consider what you need to use a mill for after buying the Victoria castings. I am always amazed at the jobs my micro mill has managed to produce, although admittedly it already has the larger table fitted.
If I had the space I would have jumped at the chance of a larger machine, but you are looking at an extra £50 just for the machine and then more for collets etc and then the workholding T-nuts, bolts, holddowns etc is more again.
Quite a lot more expense, for more power and probably more rigidity but not that much more capacity.