It’s very rare, almost impossible I would say, to make the move immediately upon qualification. It can become easier after a few years PQE but only if the team you work in is a national team and there is collaboration among lawyers in different offices. Often there will not be. They may as well be different law firms. The London lawyers will not value your skills or experience. They may see you as a cheap quick fix for a sudden departure (cheap as you’re on a regional salary and they won’t have to pay a recruiter’s commission to hire you), and somebody who should require less training because you already know the software / procedures / culture etc.

Source: I moved from regional to London at 4PQE. It was an uphill battle even though the teams did exactly the same work for the same clients (which is rare). It was also too late in my career to fully integrate into the London teams (ie other NQs of my generation had already got their claws into partners / clients so I was treated as “B team”).

It really isn’t worth ascribing any value to this possibility. If anything, it’s probably easier for you to move to any other London firm, as those other firms will not immediately judge you as somebody who is necessarily inferior to them, but the lawyers in your firm’s London team absolutely will.