We appear to have different defintions of contradictory. To me two statements are contradictory when both statements cannot be true simultaneously. This would hold for the quoted text as it states that UK time is GMT and that UK time GMT+1. The first statement is unqualified. It does not say "for part of the year british time is GMT and part it is GMT+1". Therefore it contradicts the second statement.
Also often the concept of a timezone in computers is an amalgamation of both the base offset from UTC and the daylight savings rules. GMT has no daylightsavings rules but Britain does. Therefore in this context Britain is not in the timezone GMT but rather in the timezone Western European Time which has well defined rules as to when it is WEST (Western European Standard Time aka UTC) or when it is UTC+1.
The tendency of English speakers to use "GMT" and not "UTC" is a historical quirk. UTC replaced GMT quite a while ago. So to be really pedantic Britain hasn't "been on GMT" since GMT was replaced by UTC.
PS: Some interesting/useless trivia: days used to begin at noon. Sanford Fleming is the father of the timezone (he designed the timezone system in use in North America and was one of the main instigators of the International Prime Meridian Conference in Washington in 1884 where international standard time was initially adopted) . Harrison is the father of modern Navigation and of the precision Timepiece. 1 minute of longitude is approximately equivelent to one nautical mile. Your precise longitude can be determined by setting a watch to local noon and calculating the difference to UTC. In the early days of navigation French Maps were often in PMT + X where X was the difference between the Paris Meridian and the Greenwhich Meridian. This was because the British Navigation Maps were generally higher quality (thanks to Harrison and the Royal Navy) but French Pride prevented them from explicitly stating the maps were based on the GMT and actually recalculating them all based on PMT was too expensive. The French got the last laugh later on by preventing universal time from using an English acronym, but the English got their own back by preventing unversal time from using a French acronym, until finally there was a compromise and we ended up with the non-acronym UTC.