... which must be available in the UK cuz I just saw Nigella Lawson using such a thing on her show Nigella Bites!

It's a solid unripened very young cream-based cheese, usually in a "brick" or a tub, foil-wrapped.

A neufchatel-type cheese can also be used.

ps: the Kraft Philadelphia style is not necessarily the best but it's certainly the most-used cream cheese for a basic cheesecake recipe as known here in North America.