How to thicken heavy cream without changing the flavor

Heavy cream can be frozen but only if intended to be used in its liquid form (soups, sauces, etc.) It will not whip properly once frozen.

I agree with Pointy that there's really no point in freezing it because it's usually dated about 2 months out from the time I'm purchasing it. Additionally, if kept cold and not left out on the counter unnecessarily (as with most dairy products) it will keep well beyond the date on the carton.

I'm not a milk drinker but use it baking/cooking. If I have a recipe that requires milk I use a 50:50 ratio of heavy cream and water and haven't had any issues yet. If I was using milk it would be whole milk so it's kind of like making your own version.

Corn starch only thickens when heated to 180 F, so it probably is not helping at all with your whipped cream.

I live in the US, so I cannot compare to whipped cream in the UK. Whipped cream for cake fillings is often beaten almost to the breaking point to make the foam as thick as possible. I assume you are whipping the cream sufficiently, and it still isn't getting as thick as you would like.

If that is the case, you might try stabilizing the whipped cream with gelatin. The link is one of many I have found on a quick search, and it specifically calls for 35% cream.

This long chowhound thread also has some interesting thoughts and advise.

This ochef link also provides an interesting method of using creme fraiche.