Please explain about double cream - as far as I can glean from the internet, it's supposed to be 48%. In Canada, the double cream (which is not readily available) is something you can stand a spoon in. Some of the recipes in the books call for double cream, but it sounds like it is more fluid - like whipping cream? Can I use this as a substitute if I whip it first? Thanks!
British double cream does indeed have up to 48% butterfat and it is difficult to find outside of the UK. In Canada whipping cream is the closest equivalent (in the US it is heavy cream) and has 32-35% butterfat but we would suggest trying to get the highest fat content possible. Canadian cream tends to be heat treated and has added stabilisers which can sometimes slightly affect the taste of the cream vs British cream, but is very unlikely to have an effect on any recipe.
Most of Nigella's recipes are re-tested using heavy cream and should work with Canadian whipping cream but with recipes that add liquids/alcohol to the cream before whipping - such as the no-churn ice creams - we would suggest that you whip the cream to soft peaks fist then whip in the additional liquid in a thin stream otherwise you may find that it is difficult to whip the cream. Also you my find that if cream is added to a sauce then it may be very slightly thinner than a sauce using double cream, but you can always boil the sauce (as long as it doesn't contain egg) for an extra couple of minutes to thicken it slightly.