I have to agree, it's very unusual for a lease (of all things) to prevent full staircasing. I've only done casual research for my job (mortage company), but I haven't heard of anything like this. I was also under the impression that most shared ownership mortgages required the ability to full staircase. I would double check this as alloveragainandagain suggested.
I believe the only way to release this equity would be to remortgage, but once again, I'm not too sure a mortgage company would be happy with this for a shared ownership where the purpose wasn't to buy more shares.
You are also looking to find a better rate of return that the mortgage rate you would be paying after releasing this equity. The suggestion of the investment property would be the best rate of return, BUT... the profitability on BTL is going to be hammered soon. I won't bore you with the details, but the rent received will increasingly be counted as your income, possibly pushing you into a higher tax bracket and wiping out any return you may be making.
Stocks and Shares have been fairly volatile, even funds have been a bit up and down recently. You are also putting any capital at risk, particularly if you are talking about doing this in the short term.
You may have to come to terms with staying put or moving to a better market (I've had the same realisation myself in the past couple of years). Otherwise you are looking at trying to increase your equity by overpaying (I'm assuming you have a mortgage), or hoping that your company offers you a massive pay raise... we can live in hope!