I'm so sorry for your losses, it's such a horrible thing to go through.
We had 3 miscarriages and I'm currently 25 weeks with this pregnancy. We're in the US so accessing support is a little different, but after our third loss we went to see a reproductive endocrinologist for testing. There are a lot of things that they can look into which cause or contribute to miscarriage risk. This includes anatomical issues, parental genetic mutations/translocations, endocrine issues, poor quality eggs/sperm and autoimmune issues. They can find a cause in around 50% of cases, but the good news is that the majority of people with multiple miscarriages do go on to have a successful pregnancy eventually.
In my case, I was already diagnosed with and medicated for Hashimoto's and PCOS which can both contribute to losses. Everything else came up clear and our RE expected we had an egg quality issue and our losses were chromosomal. Our options were to keep trying unassisted or start fertility treatment (IVF with PGS). We decided to give it one last unassisted try, but do a 3-month supplement regime that's been shown to boost egg quality before we started. This pregnancy happened in the first month we started trying again. I can't say the supplements did it, but I am glad we took the risk.
If you struggle with NHS timelines and you can afford it, I'd highly recommend seeing a Reproductive Endocrinologist privately for diagnosis so you can rule out the most common stuff. You'll get there eventually on the NHS too though and the UK has some amazing recurrent loss specialists.
The supplements I took are outlined in the book 'It Starts with the Egg', which is worth a read but only really applies if you fall into the category of having repeat chromosomal losses, which is what cases most non-viable pregnancies. I also highly recommend getting some counselling if you can as recurrent loss can be such a devastating and isolating experience. There is a potentially useful Facebook group called Recurrent miscarriage / multiple miscarriage support which you might be useful too.
Once more, I'm so sorry that this has happened to you.