in the US, R = Restricted = Under 17 requires accompanying parent or adult 17 or older with photo ID.
So in the UK it would be (18) - Suitable only for adults. No one younger than 18 may see an '18' film in a cinema. No one younger than 18 may rent or buy an '18' rated video.
I thought R18 is only used for specially licensed cinemas.
Sorry - the above, apart from being incorrect, is too simplistic an answer.
There is no direct equivalent sincethe MPAA "R" is "Restricted. Persons 16 and under are not admitted unless accompanied by parent or adult guardian" which overlaps the BBFC "15" and "18" ratings.