It's a problem for this sport/hobby
What you have in South east asia (herein reffered to as china, but Japan, HK and taiwan are also major players) is a whole load of factories making rifles. Some factories make several brands worth of rifles (one factory produces the VFC line and some UMAREX models) The factories are not usually tooled up for a single model for very long, maybe 2 weeks tops? you make all of the modelX then all modelY etc.
These guns are then shipped either internally (in Japan, they look after their own crowd first) or usually to HK - almost all airsoft stock passes through a single street in HK. Buyers from around the world then come and select what they want. By this time the factories are making modelZ with no intention of remanufacturing models M through Y for another year or two, once stock is dried up and the 'shiny new modelA' excitement has died down (takes around 18 months for people to stop being excited)
By the time the rifle gets to the UK/European market It's gone through the manufacturer, sometimes a wholesaler from them, then HKAIRSOFTSTREETAWESOME, then a european buyer, then a distributor and then the retail store.
Lots of links means lots of mix ups and cocking around, which means poor supply (especially to a small market like the UK)
Basically. if you want a specific rifle, buy it.
If it's TM and it's not in stock, contact LWA, Firesupport or (if all else fails) Zero one.
If it's ASG, you're usually in for a 2-3 week wait, depending on stockist
If it's cybergun branded then AHAHAHAHAHAHAHAHAHA WEE FREENCHIIEZ DO NURT CARE FOR YUU EENGLISH PIG DORGZ.
That's essentially the state of affairs. Not great, but at least we're not Germany.