OK Barky. First of all, most forces will not give you a ticket for fox control, on a rimfire rifle. This is not law, and is something that Chief Constables take responsibility for. So, on that note, you may need to look at a centrefire for foxes and there is a whole host of different calibres to choose from.
If you are only ever going to go for foxes as the largest mammal you are likely to shoot, then as a starter rifle calibre, then a .223 could be what you are looking for. If you are really going to get into it, and start reloading, then my choice would be 22-250, but this is my personal ch