Windows 11 Home to the Pro version

I’m looking to upgrade from Windows 11 Home to the Pro version. On looking at the Microsoft store the cost is £120. A quick search took me to /theunitysoft.com/ who are selling the full version (not upgrade) for £19.99. I have in the past purchased keys from such vendors with no issues.
Given that I just wanted to upgrade my existing windows 11, is there any reason why I shouldn't go for the cheaper full option?
At Microsoft’s activation section, there is an option there to upgrade using a key, would this work ok with the cheaper key that I am considering?

I've never heard of that site. Using the cheap keys is a hit and miss. I've known of several who got burned by them. Even on some of the better known sites that sell them reviews are mixed so I say use at your own risk. Here's a link from stacksocial which is a sponsor of this forum.

There are many sites selling Win 10 pro key for very little.
As Win 10 and 11 share same keys, you can use the Win 10 pro key to upgrade the Win 11 to pro.

Thanks for your replies. I decided to email theunitysoft and asked what method did they use for installing, the reply was we'll let you know after you make the purchase. I then came across a supplier called PC Keys and they stated on their Web page that activation was made via windows settings and activation. I decided to purchase and everything worked. This was the proper upgrade and cost £25.

The differences between Home & Pro are minimal to a home user.
Home lacks BitLocker disk encryption, Remote Desktop support, group policy management, and a few other advanced security and business tools.
You will probably never need any of these.

Not worth the cost to upgrade.