There is zero basis for your assertion that majority of immigrants weren't able to find jobs their skills aren't transferable. This affects certain professions to be sure, however plenty of people like software developers have skills that are very much transferable. Drivers are able to find jobs because those are the kinds of low paying jobs immigrants get, not because their other skills weren't useful. And yes it very much does boil down to opportunities available, which in UK are slim to none even for people born there.

If you're going to claim that lack of transferable skills was the core issue, then do provide a source to support your assertion.