I've never really been too bothered with 4g as most areas I'm in have pretty poor coverage but in case that changes, I wanted to check something on my iPhone 5 - is there supposed to be some sort of setting for it? It has an 'enable 3G' switch obviously but nothing relating to 4G or LTE. I'm on a Virgin Mobile contract and they don't offer the service yet so it's not like I can go somewhere with coverage and try but when they do activate it I'm assuming it'll automatically connect?

I'm pretty sure with iPhone 5 you need to be on Three or EE for 4G.
The Enable 4G switch only appears if your network adds it via a carrier update.

The iphone 5 can only handle the frequency bands that EE are using for 4G in the UK,
so it won't work for 4G via Vodafone or O2 (not sure about Three) who are using 800MHz
(liberated from TV use by Digital Switchover)

iphone5*s* has full support for all 4G UK bands

Yes but OP is on Virgin which is an MVNO on EE so I would imagine that they will be able to offer it at some point (and at the 1800Mhz the iPhone 5 supports).
When / if this happens then Virgin will send out a carrier update which will change the Enable 3G slider to an Enable 4G slider as has been said.

Cheers all - that's what I kind of suspected.

5c will work on all networks also.

The 5 works on EE and Three. I have a 5 myself and an a Three customer.

I can confirm that the iphone 5 works on Three 4G, I had the 4G symbol pop up when I was in Birmingham last weekend. And as above, your network provider will send out a carrier update which when installed will replace the 3G button in your settings

The iPhone 5 will only work on EE 4G and Three 4G as they both use 1800MHz frequency to broadcast their signal.

It wont work on O2 or Vodafone 4G as they don't broadcast on those frequencies.