I usually have an IP address that starts with 90.xxx etc or 94.xxx from the UK, but lately I have noticed that they have begun with 2.xxx or 5.xxx etc which I've never seen before. Is this/they a new prefix for the UK ?
Relatively new, yes, 2/8 was given to RIPE September 2009, and 5/8 was given to RIPE in November 2010. There will subsequently be a year or two before they actually start giving out those addresses to the ISPs to allocate to customers.
So yes, they do have a bit of new smell to them. But nothing to worry about, they work just the same as any other IP.
Where possible, contiguous blocks of addresses are allocated to the same region to allow for route summarising in the core internet routers
Internet routers these days are beefy, not really a current concern. Plus, with the fact that IPv4 is running out, you can't have too many luxuries like that any more.... sure the big blocks will be given to the local registries to allocate to ISPs, but the registries will become more stingy and give out smaller allocations.
You can tell where (almost) any IP address is
Will become more tricky in years to come with the advent of v4/v6 NAT operating at ISP level etc....