There's very little class C in the UK anyhow, class D however you'll meet loads of, and so far as I know they're basically treated the same in this regard (I'm cautious with this statement because I really can't remember when I last flew in class C in the UK).
IFR clearances are on a route and an altitude or flight level. VFR clearances *may* be on a route, or may be "own navigation remain at least 2 miles clear of X" or variations thereon. VFR clearances are never at a specific altitude. They are either "not above" or "not below".
So, for example it could be along the lines of "clear transit controlled airspace through the XXXX overhead not above 4,000ft", or "clear transit controlled airspace, remaining south of town YYYYY, not below 2,000ft".
The assumption - which is a pretty sensible one when you analyse it is that you need to be able to modify your altitude in order to maintain VMC, and thus VFR, in British weather most days. Also so long as they know you're not going to get in the way of any IFR traffic, ATC have got enough to do without agreeing every small change you make with you. If you have any reason to go outside of the cleared limits (for example, because you want to climb over or descend under some cloud that's in your way and conflicts with your altitude range clearance) that's when you ask for a new clearance.
If your licences permit you to choose between IFR and VFR, there's a black art in deciding whether to ask for an IFR or VFR clearance through CAS. If it's a big lump of quiet airspace, I'd tend to ask for a VFR clearance that gives me flexibility and keeps everybody's workload down. If it's a busy confined bit of airspace, I'd tend to ask for IFR as it's much easier for the controller if they have that much greater control over my height and route.
I don't think you'll find it in CAP 413, but it's sometimes helpful to give ATC the choice, viz "request crossing clearance VFR, but can accept IFR if this is helpful to you". I use variations on that wording quite often, where it looks like I can make the controller's life easier, and the bandwidth is quiet enough that the lengthy transmission doesn't cause problems in itself.
Hope that helps.
Last edited by Genghis the Engineer; 19th Jul 2017 at 09:43.