solo travel in england/ireland or netherlands/germany?

I expect you'd find cooler July weather in the British Isles than on the continent. Another consideration for a sociable solo traveler might be the local language(s) -- you'll have easier conversations in places where English is generally spoken. I think you'd find plenty to do, and plenty of budget lodging, in either area.

Two of us have hiked and bicycled extensively in both England and Ireland, although we’ve still barely scratched the surface. A solo traveler could do likewise, and going it alone, might even cover a lot more ground. You’d likely run into like-minded individuals, for comparing notes, and socializing when you wanted.

Scotland should be included as a worthwhile outdoor location in the British Isles for similar activities.

these days nobody can guarantee what weather you will find anywhere. Depending on where you are in which country when the hot weather strikes you can get hot in Scotland, Germany, the Netherlands or England.

Then again both the UK and Germany have many climate areas, and the weather in Munich can be very different to that in Aachen or Hamburg or Berlin.

Where did you have in mind in Germany?

There are very few places in Europe that won't go above 80F in July. It is definitely going to be the case in Amsterdam and London. In Germany, if you are in a city, it will go above 80F, but if you are in the mountains, then it won't. I honestly think that Ireland is your best weather bet, but it will also likely go above 80F at some point during the month. If you really want low 70s, then you need to go further north.

I have no idea how hot the mentioned temperatures are, but if you don't like heat July is a bad time to visit Europe in general and you might need to go a bit further north than England/Ireland. Also, weather is hard to predict and there might be a heat wave which will mean high temperatures all over Europe. The difference being it will be hotter further south. I was in England the summer before the pandemic and there was a few really hot days with the temperature peaking at 38°C, which was really hot, but at the same time it was 42°C in Paris so it could have been worse.

For reference 80F is 26C. 70-75F is 21-24C.

Several recent summers have seen both London and Paris sweltering as the temperatures reached high-90s (30 celsius). Maybe less so in Dublin. But there's another factor: Summer showers. Ireland is green because of the rain and warm wet is still wet enough to affect tourism. Relax and take a flexible set of clothes, footwear and a protective hat.
As to mixing and mingling, I think you will find company on the hostel circuit from all over rather than just natives. And Google is your friend for local activities.

Over a couple of summers when there has been a heatwave in southern England pushing 40 degrees, I have been in Pembrokeshire, South Wales and in Scotland and have been wearing a fleece both days as it’s been cool - about 15 degrees. At this time, London was warmer than much of continental Europe, so you can’t generalise.

Scotland and South Wales would both suit your interests and offer better value than London. Language wouldn’t be a barrier and there’s plenty to see and do.

I like to know how bad the temperatures can get in the places I'm heading. (For me, "bad" can be on either end of the temperature scale.) I use the website timeanddate.com to see actual, historical, day-by-day weather statistics going back about ten years--though I usually just check the most recent five--at my planned destinations. Here's Berlin for June 2021. Since your trip is toward the end of the month, I'd check both June and July. And definitely go back a bunch of years.

If you're concerned about having a 75 degree F. Summer trip, you'll need to either change your trip's dates or go to the far north in Europe--like Scandinavia.

For a young adult, Germany and the Netherlands might be a much better fit for you. Unfortunately Munich is the happening place in Germany and it's on the southern end of the country. The Austrian Alps are 2 hours south, and it's quite nice weather there.

Needless to say that sitting in a cafe in Amsterdam watching the tour boats go by while sipping a Heiniken makes for a glorious Summer afternoon. And sitting at the Konesbacher beer garden alongside the Rhine River in Koblenz, Germany on Summer afternoon is a memory still dear to my heart. The English Gardens in Munich is another eye opening experience in Summer too.

The world has so much to offer you, especially if you'd be a little tolerant of temperatures.

The weather part is a futile quest. 20 years ago, we had a week in the Lake District (a wet place) and it was hot and sunny every single day. We had a week in three Scandinavian countries (around 1990, maybe) and it was sweltering, with air conditioning non-existent. More recently we had a sweltering two weeks in Germany, and three A/C luxury hotels we stayed at could not keep up with the heat in the rooms, and we slept in our underwear. (Their system was not engineered the way they are her, I think.)

Climate is what you expect, weather is what you get.

Way back in the 90s i made a bus tour thru Ireland. The company did the tour with two buses, one traveled clockwise one counterclockwise. Staying first and last night in Dublin. Our bus had 2 times a really light sprinkling of rain, the other one heavy rain almost daily over the time of 2,5 weeks. So ask me and i say Ireland has nice summers and almost no rain, ask someone of the other bus and he will tell you the opposite.