You should be able to get Castrol M in Europe because Castrol started in England. BTW, the Castrol name comes from CASToR OiL which is what the company started with about 100 years ago.

Medicinal castor isn't recommended because it may have additives in it like sugar, in fact the Merco instructions specifically state not to use medicinal castor. Any castor labelled as AA is fine because that means it's only made by pressing the oil and doesn't have the extra step of chemical extraction which leaves traces of those chemicals that cause the white flakes sometimes seen in certain fuels.