Former Warsaw pact member states have well trained young people with degrees and horrific employment levels - so employers in the UK can get cheaper staff.
I think you mean there are "only so many talented British / English developers" who will work for what you want to pay.
Over time a shortage and higher wages will increase the supply - all those cute triplets with triple A levels that the papers like to picture jumping in the air on results day( a common uk trope) will go into engineering instead of the law, medicine or other higher paid and higher status jobs
There's only so many talented British developers. Hiking salaries won't turn non-talented people into talented people.