Is your gun empty and the bad guy pretty damn close? F**k retaining the mag, drop it like it's hot, get the gun fed and do your best to live, think about picking the mag up if and when you have the chance. As distance and available cover increases you've got more time and opportunity to retain the empty during the reload process. With an exception to be noted momentarily, dropping empties has never been official UK doctrine afaik whether with rifle or pistol, even doing CQM for deployments (not that I've seen every service/unit's range SOPs by a long shot), but the fact is it's fast so do what you gotta do if that time comes is my stance - which for me it almost certainly never will but I sure hope I come through if it does.
I don't know the details during the SLR days, but basic evolution of british ammo feeding the last 100 years (primary rifles only) has been Lee-Enfield 5rnd clips -> FAL 20 round mags -> RG 'disposable' paper thin aluminium 30 rounders -> HK Steel very-much-not-disposable 30 rounders -> Magpul 30rnd EMAGs. As mentioned we tried to copy the US in the 80s but bureaucracy caused an issue with the L85A1 and those cheap, weak mags were a large factor amongst the many problems that rifle had. The magazine is one of the most vital links in the chain of reliability for a firearm.
Dump pouches have never really been the most popular thing for stashing empties for militaries anyway if you take a holistic look at things. I've never personally seen one that was standard issue for us, the yanks, canada or anyone in Europe, I'm sure they exist but I've never seen one on anyone on ex, in theatre or in a deployed picture anywhere. I'm quite sure there's various exceptions, but it's not the norm. They're pretty good for airsoft, they're good for just stashing 'stuff' you need to shove somewhere right that second, saw a good bit of use for 'SSE' in the middle east for some cooler blokes than me, but they have a lot of drawbacks and aren't all that necessary. I'd call them more a result of the 2000's explosion in more modernised and high-speed shooting courses available for civilians in the US and the gigantic growth in the tactical gear industry that was going on at that same time.