You can use ericaceous if you like, or multi purpose/multi purpose mixed with John Innes No.2, or any combination of these

If it really is a tree, rather than a shrub, wouldn't it be better in the soil? It'll outgrow a pot.

spritz, they're sold as "trees" even when they're tiny, and even if they're only small varieties, so I'm assuming it's not, for instance, Acer griseum...

As you are in Devon and have more acidic soil (I assume but you may know different) you might be ok just putting it straight into the garden without extra compost and then topping up with some ericaceous feed? My friend in Plympton has soil where all the acid loving plants are flourishing, so I think an Acer would do well too.