Tanning quickly using 50 SFP

Here's a frustration: sunblocks do not seem to work. I have high melanin levels in my skin. Today, I was gardening for two hours in the afternoon sun. The ambient temp was around 20oC, but, yes, I was in full sun for the two hours, no hat, in a sheltered London garden. Before starting I slathered on SPF50 (full spectrum) from a in-date new bottle and I wore a long sleeved shirt. Taking a shower in the evening there was a clear dark tan line not there in the morning. My face was pinkish.
I get that we need to reapply regularly and sweating or exercise can reduce its efficacy. But neither of those should be the case. So, short of wearing a big hat and a scarf around the neck (which I do increasingly for much of the summer) what can be done and why should SPF on dark skin not work?
Any tanning is a sign of skin damage, even without burning. I have had radiotherapy and need to be careful on face, arms and upper body. I don't want any tan or skin damage. I am somewhat stumped.
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If you live in Derbyshire, as I do.
Obviously much of the skincare industry is snake oil so it’s all about what you find works for your skin, but I’m quite happy with it in terms of non-greasiness, no whitish residue, and the fact it doesn’t sting my eyes. I’m already quite tanned so don’t notice much its effects in the anti-tanning department — I can only say now that you’ve brought up tanning I don’t think I get more tanned out in the sun with it on…
“PA is used in Asian countries like South Korea and Japan. PA means Protection Grade of UVA. UVA rays penetrate the skin deeper than UVB rays and cause skin aging. Also, UVA are the predominant tanning rays.
According to the Japan Cosmetic Industry Association, PA+ corresponds to a UVA (PPD*) protection factor between 2 and 4, PA++ between 4 and 8, and PA+++ more than 8.”
https://rafaeldiascosta.wordpress.com/2017/05/27/review-thank-you-farmer-sun-project-light-sun-essence/
If you live in Derbyshire, as I do.