I've lived abroad for decades, like nanna8, but have !earned to live without many of those things. For example I still have a half bottle of Dettol which I've never used, although there is no equivalent here in Germany.
Branson pickle and Marmite I do use regularly and can order these through Amazon, who take care of the hassle of customs duty, or I get it when over in the UK, or friends bring them.
I have found tea here which suits my taste, so no longer bother with huge packs of PG Tips. Custard doesnt come into the equation as, as I have so often bemoaned, German DH has a horror of hot puddings.
Coleman's mustard was only served with roast beef when I was growing up, and I don t miss it, even when I do do a Sunday roast and we never had brown sauce with anything. My mother was a plain, albeit very good cook. We did have Salad Cream but surely no one uses that any more anyway!?!?!