It's hard to say. It could make Britain turn inward for labour. With the lost of less expensive external labour, the local market could rise as wages track demand. At the same time Britain may be loosing a lot of money soon. This could drive down demand since the EU will probably look to exclude the UK from contracts as a punitive measure, even if it's just a subconscious thing.

You might also see tech leave the country. Selling in the EU will be more costly. As a result, companies might start moving out or collapsing. Their former employees would then leave too.

From what I see, London's tech industry will largely shift to Berlin and other places.

I heard from Adobe that they will close their headquarter near London and move to their main operation to the Paris office. The UK branch essentially becomes a sales office, nothing more.

It likely means less immigrants coming in to do tech work in the UK, but there would still be huge incentives to allow some form of immigration since the UK wants to be able to have its citizens get work abroad. If they go to a more Australian model, likely alot tech work would be under a protected class of worker that would be allowed to establish some kind of residency.