There is no reason to think that generic imatinib will be any less effective than Glivec.
In order to be approved for use in the EU, a generic manufactuer must do two things - show its pill has the same active ingredient, and show the pharmacokinetics are the same (I.e the same amount of the active gets into your system in essentially the same time as is the case with the reference drug, here meaning Glivec). What generics don't have to do is all the phase 1, 2 and 3 studies that the reference drug has to go through for first approval. There is no need to do that if the pharmacokinetics are the same, and it's the same active. The generic can "piggy back" on the original data because it is essentially the same thing.
Imatinib is what's known as a small molecule drug - it's a chemical compound (as opposed to a biological). As such it's pretty simple to copy, and no doubt then formulate. I can't think of a good reason why a generic approved in the EU should work any less well than Glivec.
We're all used to taking generics in other areas and think nothing of it. Of course, imatinib isn't used for headaches and naturally patients (myself included) have doubts because of this context. That is entirely natural though I do feel my own doubts are irrational. My generic omeprazole - also a groundbreaking drug although in a rather different area - works just fine.
Ultimately, whether it's the NHS or a private payer footing the bill, a shift to generics is inevitable, and necessary. Otherwise we will find treatments - especially new ones - won't be available. Once imatinib goes generic I would expect the guidance on its use to change. I find it hard to imagine that general policy on generic substitution will not apply to imatinib.