How do I get a high pedestal toilet for downstairs cloakroom?

https://toiletfound.com/tall-toilet-tallest-toilet/

I prefer to use a riser with handles, however, here are some other considerations when looking for a Tall Boy version

https://www.thespruce.com/dont-buy-the-wrong-toilet-327008

Just buy whatever brand you can afford in "comfort height ". I recommend the Toto brand. Great toilets, kinda of exspensive.

Toto, in comfort height. Research various models, including consumer reviews, before purchase. You'll need to know your allowable dimensions, as well.

Toilets for handicap restrooms are taller

There is site that provides good information about your needs. So if you are interested, you can check this toiletsguide site.