I work in Change. My firm's research indicates that the two leaders in the UK (in terms of reputation/brand/Blue Chip contracts) are consistently Mercer HR and Towers Perrin. Performance on most indicators is pretty close.
Then other firms like Hewitt and Deloitte and smaller niche players are fighting neck-a-neck some way back, especially as the market has become more competitive and they have started cannibalising each others' clients.
Out of the firms mentioned above, in my experience Towers Perrin tends to pay best for consultant grades in HR/Change. Deloitte plateaus HC consultants in the low 40k's. Mercer and Hewitt benchmark salaries against Watson Wyatt et al. so the structure only pays out if you have an actuarial or pensions qualification.