I think this is quite a difficult question to answer. You main challenge will probably be finding a role rather than worrying about a salary. Entry level positions are very hard to find especially in London given the amount of candidates with both experience and qualifications.
Most people will probably start off temping or taking other short term or tangent work to get into HR roles.
At a guess around 20-25K for a decent job probably slightly less to get your foot in the door in London
Probably worth you looking at Reed online or other job boards to get a better feel
