I've had to do something a bit similar to this myself. The way I'd look at this is that US, UK and Belgium are easy, in that a wide range of hops, yeast and malt from them are all readily available. Ireland is a bit more of a constraint, and Switzerland is going to be almost impossible.
So you start with Switzerland and then Ireland, and fit the others round them.
The obvious answer for Switzerland is Fermentis S-189 yeast, from the Hürlimann brewery in Zürich. Yes it's a lager yeast, but you can ferment it at ale temperatures just fine. I guess otherwise you could think in terms of Alpine herbs like gentian, or adding slugs of something like Appenzeller, but S-189 is the obvious choice.
That knocks out the option of Irish and Belgian yeasts, but that's OK. Ireland does produce malt - you can get
Minch malt from the Homebrew Company in Ireland, which a lot of British brewers use. They went through hell a few months ago in the aftermath of Brexit, I assume things have settled down by now. Or perhaps something like bog myrtle (aka
Myrica, sweet gale). But if the two most difficult countries can provide you with a clean yeast and pale malt, then you're laughing, you can make almost any style. Alternatively, you could nominate Ireland to dictate the style, either a dry stout or red ale, or at least represent it with roast barley which is the distinctive ingredient of Irish stouts.
Malts from Belgian companies like Castle and Dingemans are fairly easy to find in the UK, perhaps the most distinctive is Special B, which might be interesting if you're going the dark beer route but you don't want to overdo it. Belgium does have a small hop industry and you may be able to find their hops, but they are generally just English or German varieties and not particularly exciting. The other obviously "Belgian ingredient" is candi sugar which eg
the Malt Miller has a good selection of. And even without Belgian yeast you can have a Belgian influence on the recipe with wit-type ingredients like coriander and orange peel to take it in a
Blue Moon kind of direction. Or you can claim you're aiming for De Koninck, which is effectively a golden ale made with lager yeast - and could act as cover for something heading in more of a Vienna lager kind of direction, or could equally be used as a base beer to shove a load of US hops in to take it in a West Coast IPA kind of direction.
Hops are the obvious representative from the US - given that we're aiming for 80% European in this beer I wouldn't go for big personalities like Citra. I'd go for something more subtle like Mt Hood, or just use a US hop like Bravo or Columbus/CTZ for bittering, or something old-school like Cascade just to give a bit of an American edge.
And then the sky's the limit from England - there's a good chance of getting it down to county level with eg Yorkshire and Wiltshire represented by Fawcetts and Warminster malts, you can specify Goldings from either Hereford or Kent, and so on. Also don't forget the biggest component of any beer, the water. Aside from the water physically coming from England (use Tesco Ashbeck if you're in one of the Celtic nations at the moment), you could aim for a Burton or London water profile if appropriate. Even without being able to use English yeast, things like crystal malt and Goldings are distinctively English and can go in almost anything.
So I'd say your main options are :
Guinness-y with roast barley and Special B
Blue Moon-y
De Koninck-y
De Koninck-y plus lots of US hops
Some kind of gruit with gentian and bog myrtle.
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