Just checked (in BNF - I'm not weird, I'm doing a course...) and you can get Seroquel XL over here. At a guess, because it only comes as brand and not generic in that form, doctors are less likely to prescribe it as first line because of the price. If your psychiatrist agrees that there's a clinical need for it though, he should consider prescribing it.
I'm not 100%, but from what I've heard, clozapine is also pretty sedating as well as having weight gain (similar to olanzapine) side effects and the need for regular bloods, especially to start with. It is a very effective drug, but kind of a last line because of the implications of taking it.
I wish I knew what to suggest. There are a lot of APs out there, both typical (older type) and atypical (newer ones.) It all depends exactly what is being treated though - which symptoms are being targeted. Would it be worth speaking to a MH pharmacist? They might be able to come up with some suggestions and your trust should have one.