Maybe the rest of you are smarter about this than I am, but despite numerous trips to Europe, I always forget that even England may only be in the 60's in June. If I hadn't taken my rain jacket because I checked the weather forecast and also used it for warmth, I would have had to buy another jacket to add to my collection of sweatshirts/jackets from locations much cooler than I expect this time of year!
One year, I was just in the process of packing the same clothes for Spain in October as I had the previous year for Bulgaria in October. Good thing I checked that weather forecast in time, since it was going to be too hot for my jeans and long rain coat.
The weather in England can be unpredictable. We spent three weeks in England in May, and I was roasting hot the whole time I was there. May 3, 4, 5th in London I was wearing a knit sleeveless top and capris to the Tower of London and museums. By the second week in May....still warm. We went to Salisbury from London, and while we were there, it was overcast and rainy but still warm. The last week, back to London. Still warm. I never needed a sweater and one fleece jacket I took. One thing I would always take to England is a rain jacket.
We are just back from Sweden, Russia, Estonia, Finland, and Denmark. Even though weather was predicted to be relatively mild, an unexpected cold front hit St. Petersburg, and we had a historical record low "daytime high" temp for that particular day in June. I never would have dreamed I would have needed gloves in June! My hands were like ice paws. Desperate times called for desperate measures, so I raided my husband's socks and wore a pair of men's REI black wool socks as mittens. Was so glad I had two light jackets (one a Target fleece zip jacket and the other a Columbia hooded light rain jacket) to layer, as it was cold and rainy several days. I never dreamed I would need to wear BOTH the same time.
And, of course, now that we are home, the high temps there are in the 60s/70s and it's sunny.
Layers are always a good idea and make for flexibility when traveling. and light layers compress easily for packing. Both of my jackets roll and fold without wrinkling.
While Rebecca was rosting in England, we wore sweaters and raincoats in several different regions of France in both May and June.
A couple of years ago when I lived in Europe, I didn't hang up my winter coat for the final time until June. And I have lived most of my life in locations with cold winters.
It happens, but isn't common.
I check climate data in preparation for travel. Average high/low temperature and precipitation data is usually easy to come by. RS guide books typically have seasonal data. The difficult data to find is the extreme condition data and weather (sic) to prepare for those extremes.
UK is the exception as to finding extreme climate statistics. Here is a link to Eastern England data:
Note that Cambridge June temperatures report June average high/low temperatures of 20C/10C (68F/50F), but June extremes of 33C/0C.
Pretty hard to prepare for extremes, but means are just averages. Travel is exciting in many ways.