Well, the two biggest barriers aren't field numbers, public image, or even perception, but much simpler but much deeper, far reaching, and difficult to solve in the UK:
Prices and longevity.
Paint prices really are at a level that cannot encourage expansion and barely affords existing levels of play. What?!? I can pretty much guarantee any company within the industry would read that last sentence and wonder how I dare question companies from manufacturing to importing to fields on their business decisions, but it's the truth really. When it costs as much as ten meals eaten out a pub to pay for one day's worth of paint, exactly who can afford to and is willing to spend that kind of money on it who isn't already playing? Nearly no one. Paintball expenditure falls in an odd demographic that lies in a very delicate balance: people who have enough money and are young enough, whether by age or just at heart, to want to play a sport like paintball and old and/or successful enough to make enough money to pay for it but aren't too old or too wealthy to want to do something else like say watch expensive sporting events, travel around a lot, buy a bigger house, or have kids or another kid if they already have some. The first mentioned will say they can't afford paintball because they don't have enough money to do anything like it, and the second will say they don't have enough money to play paintball because they already spend that much money or more on other things and don't have that much money left over, or consider themselves above something like paintball.
How do I know this? I study stuff like this for a living.
Secondly, the longevity of paintball players with the exception of the handfuls at the top isn't all that great. People who can focus on paintball either have friends and significant others that understand and/or participate in it or don't have much that competes for their time, money, and attention. If it isn't your life, it's either carefully worked into your life because of planning and understanding or not much else to do, or it simply doesn't work out. Ideally to stay in paintball a long time you either have to be single and working at a well paying job or involved with it at some business level, or have a very loving spouse that knows what you do and doesn't assume that when you have pink on your neck and $2000 less in your checkbook after the weekend that you've been off getting rough, intense workout sessions with a rather expensive prostitute. Professors (teachers, lecturers, whatever term applicable), parents on a budget, demanding jobs, mortgage payment collectors, and jealous girlfriends/wives are most often paintball's worst enemies.
I don't think anyone that knows much about paintball will tell you otherwise. And what's worse, other than politely asking certain companies to stop charging $1 for an 8 cent screw or $50 for a $5 bar of aluminum that happens to resemble a mount for a nitrogen tank that would cost $5 for anything else other than paintball, there aren't many solutions. That, lobbying efforts to lower taxes on paintball expenditures, and heavy recruiting efforts are what must be done before paintball will ever grow in the UK. You can take that to the bank.
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