I highly doubt that the UK will change clothing sizes. I say this because as it stands clothing sizes vary greatly from country to country even with countries that use the metric system. You grandchildren will probably just wonder why clothing sizes ...
Register for lots of onesies....in various sizes...u can never have to many....babies grow out of clothing so quickly and not to mention the stains that accumulate due to spit up...food..etc. I wouldnt worry about registering for all the cutesy cloth...
The sizes are typically newborn, 1mth, 3mths, 6, 9 (some brands), 12, 18, 24 (2T), 32 (3T). No sizes inbetween. You'll find they dont mean much for most children, as all children are sized differently. For instance, my daughter was born a week late, ...