In addition to quantity, safe alcohol limits must also take into account frequency.So I can drink 21 units a week, provided that I spread my consumption over three, four, or perhaps five days a week (a UK unit is a centilitre, so there are 9 units in a 75cl bottle of wine containing 12% alcohol by volume: nowadays most wine is a little stronger than that). It seems strange that somewhat uneven consumption should be safer than drinking the same amount every day: what's the evidence?
There is an increased risk of liver disease for those who drink daily or near-daily, compared with those who drink periodically or intermittently.
We recommend a safe alcohol consumption limit of between 0 and 21 units a week for men and 0 and 14 units a week for women provided the total amount is not drunk in one or two bouts, and that there are two to three alcohol-free days a week.
At these levels, most individuals are unlikely to come to harm.
The BBC report arises (indirectly) from a written report by the RCP to a House of Commons committee. The most relevent section of the report says:
Current Department of Health sensible drinking guidelines state that regular consumption of between three and four units a day for men, and between two and three units a day by women will not accrue significant health risk. Regular is defined as ‘drinking every day or most days of the week'.The report proposes that:
26. This suggestion that daily drinking is low risk runs against evidence which suggests that frequency of drinking is a significant risk factor for the development of alcohol dependency, and the development of alcoholic liver disease.
A very simple addition would remedy this problem namely a recommendation that to remain within safe limits of alcohol consumption that people have three alcohol-free days a week.Reference  is this paper, which reports most pertinently that:
A large North American study of over 22,000 also showed that daily drinking carried more than twice the risk of liver damage compared with intermittent drinking once or twice per week .Reference  therein is this paper, which investigates the health effects of frequent "risk drinking":
Risk drinking was defined as consuming the equivalent of 5+ standard drinks in a day for men and the equivalent of 4+ standard drinks in a day for women.It doesn't define a "standard drink", but it seems that in the USA a standard drink contains about 14g of pure alcohol. The density of alcohol (ethanol) is 0.789g/cm^3, so that's about one and three quarter UK units. 4 standard drinks would be 7 units, 5 standard drinks would be 8.75 units.
This table in the paper quantifies the risk of a list of adverse consequences at various frequencies of risk drinking. It shows that drinking a bottle of wine nearly every day gives about twice the risk of liver disease of drinking a bottle of wine three or four times a week. I'm not surprised - that's about twice as much alcohol consumed.
My summary is that the papers cited do not support the RCP's conclusion. There is no evidence that drinking 3 units of alcohol every day is worse for you than drinking the same 21 units a week spread over only four days.
I suspect that they've made this recommendation because they think if you drink less often, you'll drink less in total. They may well be right. But I'd recommend a more honest approach, along the lines of "most people who drink alcohol several times a week consume more than they think they do. One good way to reduce consumption is to have three days a week on which you drink no alcohol."