LONDON, June 9 (Reuters) - Britain said on Tuesday that a law to crack down on proxies acting for states deemed to be hostile such as Iran was expected to come into force next month, as it steps up powers to counter what it says is a growing threat posed by such groups. The new powers, promised in the wake of a spate of antisemitic attacks in London, seek to close a gap in legislation to target state-linked organisations paying organised crime groups or low-level felons to carry out surveillance, sabotage, or other activities on their behalf.