Just for everyone's knowledge.
I had a windows 8.1 key back from 2013. Found the key last week and directly used in my Windows 10 to activate it (Well it was activated already through different methods) and it accepted it in a second.
got notification of thanks for purchase and few more things like adding my phone number for security and all that.
Conclusion: You can buy/get a 8.1 key also to activate your windows 10.
How is the Windows 10 OEM key different from ones available at eBay UK website for less than Rs. 300?
I have bought 3 Windows 10 key for around Rs. 200/each and have been using it for more than a year, no problem
Please excuse the rudeness.