Spreadbetting is tax free because the punter is not the beneficial holder of stock (the bet merely 'mirrors' the movement of the underlying security - its a derivative if you like). If stock IS acquired by the spreadbet company, they do so at their own discretion to hedge the wager. In reality on most small and illiquid stocks, spreadbet companies always hedge their positions. Bets expire on a given date, so if the client doesn't want to 'role' their bet, i.e. continue with the same degree of exposure (for an additional fee) to the movements of the underlying security (eg a stock like GNE), and the company has a hedged position then it will result in the spreadbet company selling (or buying - if their client initially went short) their hedge into the market. Thus spreadbets can affect the price.
BTW just lurking here because I'm tempted by the long term story but can't help thinking we will get the opportunity to buy under a quid, so just watching and waiting for the moment.