You are correct in implying that British Summer Time begins at 01:00 UTC (1 a.m.), which is when the times given by the MSF radio service and the NPL Telephone Time Service (TTS) are altered. In fact, because the information is sent in advance, the MSF signal has already changed during the previous minute, and the TTS signal changes during the previous second. However, only mains-powered radio-controlled clocks are likely to be continuously checking the MSF signal. Radio-controlled clocks typically check the time only every hour or every two hours, or even just once a day, and radio-controlled watches only perhaps three times a day. This is to conserve the battery, as the reception unit takes more power than the clock or watch mechanism. So some radio-controlled clocks or watches may not 'realise' that the time has changed until an hour or more later. If you have a particular need to have a radio-controlled clock change without undue delay you are advised to check with the supplier or test the unit before buying. Clocks which check every two hours should reset to winter time (which starts at 02:00 BST, or 2 a.m. 'current' time) promptly.

Clocks using the Telephone Time Service are only updated when a telephone call is made to the service. However, the data transmitted by the TTS includes details of the timing of the next change to or from summer time so the local clock can be pre-programmed to implement the change at the right time.