There should be no problem with carrying national letters as the rules only specify "if required" and when they "shall be required". See the wording below (italics). As far as I am aware there is no rule that says they are not allowed at other events.
4(f) National Letters, if required, shall conform to the
same type, size, spacing and requirements as sail
numbers (refer rule 4(b), (c), (d) and (e)) and shall
be positioned as follows (also see diagram):
National Letters shall be required at all World
Championships, Regional Championships and
events described as international events in the
notice of race or sailing instructions. National
Letters may be required at any other regatta by
the notice of race or sailing instructions.
I've always been a little confused about the "GBR" label for Olympic competitors (or sail country codes as the case may be). Clearly from what fat-n-old says GBR is more than just England, but not all of the UK. But what about people from Northern Ireland? What country code do they use?
Northern Ireland is part on the UK and people who compete from there in Olympic and other international events where Scotland Wales and Northern Ireland are not 'recognised' as seerate countries will compete under GBR.
This is why there were around 30 years of 'trouble' with the IRA and various other factions. they did not want to be part of Great Britain.
As already pointed out the Republic of Ireland competes under IRL.
I think the only 'International' competitions that spring to mind where England, Scotland, Wales and Northern Ireland compete seperately are in football, rugby, cricket and the commonwealth games.
Peel it off and put JSH on so everybody will know you are Josh.
You will probably have more fun that way.
A few years back I was in Canada and put TEX on it for a joke. Everybody seemed to have fun with it except one old complaining dingbat from Terra Verde Florida. I felt sorry for him becuase he missed out on the concept that Laser sailing is supposed to be fun.
(Eventually I quit feeling sorry for him and switched to utter but that is a different story. )