Yes I know what you mean, sometimes you can get things via Amazon but then get a customs change, grr. However I have found some UK places to get things from. None of the following have customs charges:
I get Longvida Curcumin from Amazon UK. https://www.amazon.co.uk/Longvida-Optim ... a+curcumin
It is possible to get the folate etc as well. and B vitamins, https://www.amazon.co.uk/Super-B-Comple ... ds=igennus
and Magtein magnesium. https://www.amazon.co.uk/Source-Natural ... -threonate
With fish oils I got one from Nordic oils https://www.amazon.co.uk/Nordic-Oil-Str ... dic+oiland
I also used them for Vitamin D.. https://www.amazon.co.uk/Nordic-Oil-Vit ... vitamin+d3
I didn't get on very well with the methyl folate in the B Vits so take this instead. https://www.amazon.co.uk/Together-Healt ... b+vitamins
However methylated folate is recommended by the Bredesen book.
I found both Nordic oils and Igennus (vitamins) seem good, Igennus have nutritionalists who can answer questions about the supplements also. I just use some of the above but might not be the same for everyone (doses etc) so may want to check specifically what you want / need first, but I can recommend them in terms of delivery etc. In particular the fish oil is surprisingly nice and tastes lemony and not fishy at all! It has around the 1mg DHA recommended in the book in a large teaspoon. You keep it in the fridge although there are capsules also.
I didn't get on very well with the methyl folate so take this instead. However methyl folate is recommended by the Bredesen book.
Some of these are quite expensive and I can't afford to take them all the time, I try and do the basics as a priority (low carb diet and intermittent fasting, exercise B and D vitamins- which also are the cheapest) I have found the Curcumin to make a difference to my mental health though so going to try and keep up with that if I can. The Magtein was also good but I can't afford that at the mo, so have put on hold for a bit. Found it seemed to help with sleep.
With the End of Alzheimer's book, my library in Bath had that, or you can order it from Amazon, for about ten pounds. https://www.amazon.co.uk/End-Alzheimers ... alzheimers
I haven't found my GP was able to help me with much testing when I told them about the APOE4. They did some general tests though, cholesterol, HB1AC for me and had just have the 40 yr old NHS health check which covered some of that anyway. They just told me lifestyle changes are what to try, but not much else.
There seems to be a place which does a test for homocysteine level, (GP said was not available on the NHS) http://www.foodforthebrain.org
I haven't tried it yet though. On a bit of a budget here so might be something for the future for me.
The other things I have been thinking of is HRT and might be possible to get that via NHS. Patches seem to be best. Again in the future for me.
Here is another recent thread about simple changes which might also be helpful. viewtopic.php?f=4&t=4776
As you can see I'm not an expert here and just learning, only joined a few months ago. I'm a mum and former teacher, age 41 with two young boys to look after so my time is a bit limited. There are lots of lovely helpful people on this site and it is a positive place to be.