It's as jno says, Philtaz. Christmas is a traditional holiday period in the UK.
For many people (me included) it has little or nothing to do with Christianity, or indeed any other religion. Life in the UK very much revolves around Christian festivals. You will hear “We’ll do that after Christmas” or “We must meet up at Easter”. People no longer use these annual milestones because of their religious significance. These are times of the year when those who are able to do so take a break from work or other everyday routines. Christmas, religiously speaking, probably means as little to me as it does to a Muslim. It is the tradition of the nation in which I take part.
I have a number of Muslim friends. I take an interest in how they hold up under the deprivations they endure during Ramadan. I wish them well for the festival of Eid and present some of them with tokens of food for their feast. But Eid is not a national holiday here and it is not a time when the entire country takes a break.
If we are ever to achieve a homogenous society it is necessary to move away from the sort of “separation” which I think you are suggesting ought to exist. Just what sort of images does “a traditional family Christmas” conjure up to you? We don’t all head off to midnight mass. We don’t all stand round the Christmas tree singing carols. Christmas is many things to many people. This country has an annual routine which is partially identified by dates in the religious calendar. You don’t have to go to church and celebrate the birth or death of Christ to be part of that routine. But if you are to be (or wish to become) a part of UK society Christmas should mean something to you as an important time of year for the vast majority of the people amongst whom you live.
I rarely hold any brief for the BBC. In fact I am very surprised that they did not suggest that Mo Farah, as a Muslim, had no need to subject his family to the oppressive rigours of a “traditional UK family Christmas”. They may have used a bit of poetic licence in the same fashion that they often do when suggesting that members of the Royal Family are just “ordinary folk”. But on this occasion I believe they have got it about right.