The advantage of HS2 using European loading gauges has nothing to do with us being in Europe, rather that by doing so we can get more people on each train. Given that the route is being built because of capacity problems, scrapping it in favour of the more restrictive UK versions would be short sighted.
It may well be that over time that more lines are converted to allow HS services to run on "conventional" routes. Although expensive, especially in regards to bridges, it does bring benefits which could justify the cost. The expense could be reduced if NR had a policy that all new structures would need to be designed to take this on board.
I wasn't talking about building the line to UK loading gauge in terms of bridge and tunnel clearances, just using standard UK PLATFORM height and lateral clearance so all trains can use the same platforms on the new line. You could still have double decker trains if desired for captive services. Whilst UK platforms are slightly higher and intrude closer to the nearest rail than in Europe, all the HS2 platforms at intermediate stations are on platform loops, so UK platform standards being employed would not actually prevent full European size stock passing on the through lines. Anywhere such Euro trains DID stop would require dedicated platforms clearly, which would seem likely anyway for security and immigration concerns if through European services ever happen at all, and they will always be in the minority compared to domestic services.
So to sum up:
Build the line as planned to Euro gauge (GC I think)
Build all new platforms at Euston and intermediate stations en route to UK standards.
Purchase captive AND classic compatible stock with UK platform height and clearance standards, but captives not limited to UK classic height restriction so double decker still possible.
Build a limited number of terminal platforms at non London terminals in Leeds, Manchester, Birmingham (one or two platforms ONLY at each) for possible future conversion to Euro platform standard for possible future through continental services. Think about security and immigration segregation from other parts of those stations in that design process. Other platforms to be built to UK standard. In the meantime for the foreseeable future, provide conversion decking on the Euro-proofed platforms to bring height and reach to UK standards.
Result:
A standard railway where any UKHS train can use any platform on the system, so groups of platforms at Euston for example need not be dedicated for either classic compatible or captive trains. This brings much more real time flexibility in operations, and also long term service flexibility over the balance in services between the two types of train and how that may change over the years with the size of the network and the extent of through running to the classic network.