The reply above mine, includes a link to the online roaming costs tool.
In pure monetary terms, use of a local SIM in each country will nearly always work out cheaper than using roaming facilities.
The benefit of roaming is that your UK number travels with you, and calls & texts use that number as you travel - with the costs added to your UK bill. Clearly that has less relevance for data use, although websites you visit and apps you use, will identify you as being a foreign user rather than a UK-based user.
