In this country the weather is somewhat unpredictable. To deal with these wet situations you need to be prepared. I've had down pours at weddings in mid July and late Dec there is no real safe zone. When discussing details of the wedding day with your happy couple make sure you bring this to their attention. You can organise provisions should there be a down pour and you need to shoot inside.
It's not all doom and gloom though, taking some coloured umbrella's along can mean you get some great shots if your bride and groom are happy to brave it outside for a short while for their wedding photography.
A good tip which benefitted me In the above image with the VW Beatle and the below image at Houghton House is to charm the driver into holding an umbrella over you while you shoot so you don't get too wet.
In some instances the rain can be a blessing in disguise, for example when I was shooting a wedding in mid July on a Saturday on Brighton sea front the rain cleared the beach of people. When it stopped I took the bride and groom down to the sea and it looked like it was just them on the beach. Had it been a nice day I would never have got those images without thousands of tourists cluttering my shots.