The weather is better from June to September normally, but British weather is a law to itself and can change many times in a day, although it is rarely extreme, blustery showers being counted as 'bad' weather. Most people don't see canal cruising as just a 'fair weather' holiday, there's lots to do whatever the weather. Britain is a fairly compact country and has efficient transport systems. The northern most English canals are no more than 3 hours by train from London and if you fly into Manchester or Birmingham airports for instance you could be at any one of dozens of canalside boatyards by taxi or bus in under an hour. If you rent a boat or join a hotelboat cruise you will get detailed joining information.
Water can be dangerous and boats are heavy moving objects.
You should always take care of course, and children and those who can't swim should wear lifejackets, especially if they are getting on and off the boat frequently. However canals are usually shallow and slow moving and all operators will give safety instructions and training. Personal safety and crime are not major problems on the canals provided normal sensible precautions, like locking boat doors when unoccupied, are practised. There are a few inner city areas where additional precautions are advised, your operator will give details. It also includes some rivers which have been made navigable over many hundreds of years.