Of course or equally as valid a cheaper one from ebay. I've yet to see any evidence that this retailers keys are any different and there's plenty to doubt it. You won't have a legitimate copy, but it'll likely work.
Thanks for your reply, Ad. The PC is a custom build, with no COA attached. I just installed W10 Home from the MS installation software from USB. Could I still use this key? Thanks ;)
If the CoA is W10 Home then absolutely, why wouldn't you. If it's W7/8 Home then it's a grey area but most would consider fair use. If you don't care about legitimacy then a key for a couple of quid on ebay. If the above doesn't apply then £75-100 is the price, I'm afraid.
For Office 2019 buy "Home & Student 2019" (~£100 from a reputable retailer, eg. ebuyer, John Lewis, Amazon direct/not marketplace), "Home & Business 2019" or check if your employer offers it on "Home Use" basis (~£10).
First is a grey area and poor reporting. (i) it works and Microsoft haven't stopped it (ii) officially the offer is long over and Microsoft have said it's no longer valid Second your link doesn't work and is also a grey area. (i) if that CoA is brand new, never used that should be legitimate (ii) odds are that's a used oem licence so it's not legitimate, it's just taking advantage of the fact that the key hasn't been deactivated