It's a typical scenario in the UK. One minute you're soaking up the sunshine, while the next minute, you and hundreds of others are just soaking - rushing for shelter as the hail and thunder pours down in biblical proportions.
A great way to make money from this climatic confusion is to sell waterproof clothing. Cagoules are cheap, can be carried around in a pocket or bag, and are not necessarily unfashionable.
Due to their minimal features and disposable nature, they're inexpensive to buy - specialist outdoor equipment wholesalers are the places to get them from.
Either carry a pack of 20 or 30 with you and hang around popular resorts waiting for the clouds to darken, or simply go about your business, and sell them anytime the weather breaks.
Sell the 'make do macs' for around £2-£3 to tenacious sun seekers waiting for the shower to pass over. If you are in a really good position you could extend your range to include umbrellas.
This could be a useful addition to an existing 'high footfall' business. e.g. A café could offer them to customers as they leave.