It's perfectly legal (and has been so since 1994)
There can be no limit on the time allowed for 'browsing' because the shop isn't legally 'open'. (i.e. it's not 'open for trading').
The 30 minute period, for the completion of shopping, has always been allowed since the Sunday Trading Act came into force:
"Where a person is charged with having contravened paragraph 2(1) above, in relation to a large shop which was permitted to be open for the serving of retail customers on the Sunday in question during the permitted Sunday opening hours specified in a notice under paragraph 4 above, by reason of his having served a retail customer after the end of those hours, it shall be a defence to prove that the customer was in the shop before that time and left not later than half an hour after that time"
(Schedule 1, para 8).
Chris