I'm fairly sure that A and C (whose comment is on the preceding page) is correct and that it is, or was, a customs requirement to sign in and out. The Customs and Excise Management Act 1979 gave Customs the power to make various regulations and I'm fairly sure that a requirement to sign in and out was one of those. Whether that part of the Act survived the transfer of responsibility for customs controls at the border from HM Revenue and Customs (the successor to HMCE) to the UK Border Agency is something I don't know.