Costs are also a big difference (Score 3, Informative)

In the USA, each party pays their own costs no matter what happens. However, in the UK, if you file an action and it does not win, then you have to pay the costs for the other party. So, at the best, they can force them in the UK legal system, and because the spammer is not in the UK, they can seek the spammer puts up security for costs (to ensure they pay if they don't win). Thus the spammer would never touch the UK system as they would end up loosing the money they pay their lawyers, plus the cost for spamhaus to defend it.
ps... IANAL - anymore.