There is no backdoor. We do not have the export variant of the F35 with US-controlled software. The software on our F35s (and Trident missiles) is British. This came as the result of concerns New Labour had about the very thing you mention - US backdoors.
And I agree that the US cannot be trusted. Thankfully our sixth gen fighters have no US ties, and most of our other recent military developments aren't either. For the time being, though, we can't really abruptly scrap trident or F35s, despite maintenance not being 100% done here (particularly for Trident). Our missiles only need maintenance every 10 years - I'd hope we have facilities to do that domestically by then.
Tech firms will suggest any invasive nonsense that will make them money.
A completely unsurprising list of companies lol