There is no fundamental difference between the proposed route, and doing what Ireland was doing until not long ago which was converting an FAA ATPL into a JAR-FCL ATPL
I gather the UK CAA used to do the same, until about 20 years ago.
Not to mention various other routes that have appeared over the years. Some still exist but usually involve some hassle, like actually working for an operator of an aircraft of the desired registry.
The aviation business has always been full of this stuff. Nobody in their right mind is going to sit 14 exams and spend £30k+ doing the logbook stuffing if they can get a straight paper swap, which
according to ICAO principles should have been offered in the first place
It is only blatent in-your-face Euro job creation / job protection which has given rise to the JAR-FCL restrictive practices.
Of course, nobody actually working for an airline, especially a flag carrier, would have done any of this, when a harder route was available to them
One thing which makes me smile a little is that whenever one bumps into a bunch of private UK IR holders, it turns out that nearly all of them did it pre-1999 i.e. under the old 700hr route. Of course I never question this because I automatically assume that when JAR-FCL came in, they all went to an FTO and re-did the whole 50/55hr course and re-sat the exams
So, if you can find a route through this maze, go for it, but make sure you are a good and current-on-type pilot notwithstanding, by flying often, and practicing instrument procedures often, using checklists, etc.