The "tanks are obsolete" thing has been going on since well before I first developed an interest in them, and that was nearly 40 years ago. The UK does have a problem in that our manufacturing base has been largely trashed though I honestly have little idea how much is left. The alternative of "buy American!" that we always seem to end up with doesn't always have the best outcome: stuff often not as good as claimed (as a nation we do have a habit of talking down our own stuff while believing the hyperbole about someone else's) and the single-source-of-ammunition sounds idiotic, especially for a gun that sounds so ropey.
The rifled-vs-smoothbore thing is a controversy that will go on and on. I'm not sure how things currently stand but way back it was the case that tank-on-tank action was rare enough that e.g. the Cromwell's excellent 57mm AT gun was bored out to fire the fat lazy 75mm shells because HE was ten times more useful; if they needed something with more hitting power against tanks then a Firefly or something like that would be available (probably a bad example as they were usually assigned with other Shermans, but you get the idea). My understanding was the main reason for going down the smoothbore route was commonality of ammunition, because that idea has always worked out so well, thinking of the EM1/2 and Taden being an excellent modern weapons system of their time and having to be binned because the US Army wanted something ballistically identical to all WW1 battle rifles and of course we had to go along with it. Same story with e.g. the engines of '60s MBTs where the US and Germany said they must be multifuel then the Americans stuck with a petrol engine and the Germans with what is essentially a normal diesel, while the Chieftain coughed and spluttered with its bizarre and underpowered dual-fuel jobbie while the others looked on and said, "lol, how's that order book looking?"
Re small arms, the British Army tried more than once to replace the .303 but it didn't happen due to a mix of huge stockpiles and a lack of desire to spend money on rearming outside of wartime. AFAICT the jamming problem seems quite apocryphal and there were a lot of automatic weapons designed for it, whether via curly magazines like the Bren, Madsen, Berthier etc, belt-fed like the Vickers and Browning M1919 and others. In spite of quite definitely being an anachronism by WWII it worked well enough and from what I recall had pretty much identical ballistic performance to the Mauser 8mm. Which brings up an interesting point about ammunition commonality as locally-designed British tanks all used Mauser 8mm for their machine guns, as they were fitted with albeit British-made versions of the Czech MMG designs and someone made the decision that there was little benefit going to the bother of converting them to fire .303. And there wasn't: logistically it was never a problem. Really they needed look no further than the Vickers and Lee Enfield situation: while technically both fired the same .303 ammunition, firing Vickers .303s out of Lee Enfields would soon destroy their barrels. Which of course the squaddies did anyway as they'd heard they were more powerful. AFAIK they weren't, they just used a more streamlined bullet design to gain extra range.
Also re nukes, it's a shame that also went down the "buy American" route as we'd developed our own, having been locked out of the weapons programme for security breaches (er, remind me who leaked our Magnox reactor designs ultimately leading to fairly low-tech countries like NK being able to build them...?) which were actually better designs in some ways, i.e. being less prone to being affected by other nearby nuclear blasts. But admittedly secured by what was essentially a bicycle lock, and that slightly British thing of "er, did we say 1MT? Three is sort of about there, isn't it? Oops."