It's true, any advert at all that comes through Spotlight (actors portal where such things are advertised) will have a cast with racial, gender and age restrictions placed on the characters which between them all cover pretty much a catch all of British society- and as you can be paid a lot of money as a featured role in an advert, it's simple economics ( also time costs with adverts obviously) to have a mixed race couple rather than 2 x whole families, unless it's plotline necessary. It's done for financial reasons, to show inclusion and for time constraint reasons and to sell their product. Nothing sinister about any of it.