Couple planning ~4 weeks mid-July to mid-August (birthday trip). Would SEA be a good option from London?

My partner and I want to travel ~4 weeks mid-July to mid-August (we’ll work 2 week and travel 2 week) £1,500/person budget including flights,. My heart was set on Peru + Columbia but flights alone are around £900 so now I am considering SEA (have seen round trip tickets for £500).

We’d like private rooms/private dorms, love walking, history, culture and nature.

Being from India, I love monsoons so hopefully rainy season shouldn’t be a big bother but any country/region I should avoid? Is the budget realistic or should we look at Europe? I imagine most of South Europe would be hot and expensive during this time?