You've been offered a job that pays at least £15k less than your current role and you're asking an Internet forum for advice on whether you should accept it?
On a more constructive note, organisation hierarchies across the Big 4 are not exactly consistent, so it's hard to give an exact answer. However, in the Big 4 consulting practice I work in, a Senior Associate is a junior role which would rarely command a salary of over £50k. I realised M&A is slightly different so this yardstick might be limited in its usefulness.
Ultimately you've got to consider whether consulting is the right direction for you to go in, and if so, whether staying in your current role for longer would allow you to come in at a higher level where the pay might be more aligned.
Personally if I could get £100k in industry I wouldn't even consider consulting.