Whenever I buy manure it's usually rotted cow manure. It's always served the purpose. Your local garden center should be in stock.

Probably better not to plant them under the roses because there's a real danger of disturbing the rose roots when you dig them up. Probably better to grow them in a potato sack - some of our members do this pretty successfully. There is some pretty comprehensive advice here:
https://www.growveg.co.uk/guides/how-to-successfully-grow-potatoes-in-containers/

Pink fir apple is a great tatty, though not pretty...but its a really late variety not usually recommended for containers. But as you've got it see how you go. Another year try an earlier one.

Putting spuds among roses is a terrible idea imo. As well as the digging problem damaging the roses, you'll generally never remove ALL the small potatoes so they'll come back randomly next year.
Sacks are the best way, just use old compost bags or anything large enough & strong enough to last. Roll down the sack, quarter fill with multipurpose compost, put the seed spuds in & cover & water. As the green, above-ground, bit grows up to about a foot high, completely cover it by rolling the bag up & tipping in compost. Just leaving a few green tips showing.
Keep repeating the process until the bags full.
When the green plant bit has passed it's prime it'll start visibly dying back & you can start harvesting.
Occasional feeds are good & as you're growing in just compost the potatoes will be better shaped than in stony earth & pests should be less.
Phew! Good luck

I did wonder about the rose bit. As I have the PFA will put in tub & get an early one for a sack. Watch this space! Thanks.