Hi there,
The cashiers won’t question anything nor probably say anything. Don’t forget they probably deal with this on a daily basis so they are used to it and it’s a normal thing so they don’t judge.
Even if you are under the age of 16 they will still sell you condoms since they’d rather you had protected sex to stop pregnancy than question you and not provide you with any.
You can also buy them online on websites such as Amazon but don’t forget to get these from reputable brands such as Durex!
To sum up, they won’t ask for your age. They won’t judge you nor will anyone else. It’s completely normal (you not seeing anyone buying them is just you, ask a Boots cashier and they will give you a different opinion). If you are still worried buy online.
Hope this helps
Joe