1(1)It is hereby declared that a person is not to be treated, for the purpose of the exemption in section 3(4)(a) of this Act for persons ordinarily resident in Great Britain, as ordinarily resident in Great Britain at a time when he is there in breach of—
(a)an exclusion order ;
(b)the [1971 c. 77.] Immigration Act 1971 ; or
(c)any law for purposes similar to that Act which is or was for the time being (before or after the passing of this Act) in force in any part of Great Britain.
(2)In that exemption " the last 20 years " is to be taken as a period amounting in total to 20 years exclusive of any time during which the person claiming exemption was undergoing imprisonment or detention by virtue of a sentence passed for an offence on a conviction in the United Kingdom and Islands, and the period for which he was imprisoned or detained by virtue of the sentence amounted to six months or more.