the fact that London has ceased to be the financial capital of Europe.
As of March 2020 London is rated #2 in world financial centres (behind New York). The closest European one is Geneva at #9, and the closest EU one is Frankfurt at #13.
what benefit -- in pounds and pence -- is Britain gaining from having left?
Little yet, of course. Brexit isn't a magic bullet, it's an escape from the constraints that are dragging the EU down to a stagnant, mediocre, economic polity. UK growth has always been better than the Eurozone, and will continue to do better, but it will take years, of course, for the benefits of Brexit to really take hold. That's always the case when a country gains independence from a controlling bloc.
Brexiteers claim that Britain will not be subject to foreign courts, but just a lie: if we want to trade, foreign courts will be part and parcel of any deal,
You misunderstand (or misrepresent) the situation. The issue is one of jurisdiction. Of course EU courts have jurisdiction over EU issues, and of course British trade with the EU will come at least partly under that jurisdiction and those courts. That should be obvious, no-one has ever claimed otherwise.
The difference now is that EU courts will not be able to overrule British courts on purely British issues.
Also EHCR
Both the European Convention on Human Rights and the European Court of Human Rights derive from the Council of Europe, not from the EU.The UK was a founding member, and is still a member, of the Council of Europe (which is much larger than the EU).
ITLOS
Not even a European body, it's the International Tribunal for the Law Of the Sea, and irrelevant in the context of Brexit.
Lastly, the claim that "we're never going back" is suspect and inherently naive: Northern Ireland is the closest to "going back", but the Channel Islands and Scotland both recognize a different balance of probabilities...
Since the Channel Islands were never in the EU in the first place, the question of going back doesn't arise. If Scotland were so foolish as to leave the UK and (re)join the EU I don't envy them the problems of policing the border. With NI there is an incentive for the UK to find a suitable accommodation with the RoI to keep an open border, but no such incentive would exist for England with respect to Scotland. They could just throw up a fence & leave the Scots to sort it out.