I used to work at BDO, and we always promoted ICAEW above ACCA. Besides being more prestigious, BDO also offered full sponsorship for staff who wanted to pursue their ICAEW qualifications. And as mentioned earlier, there's no point getting more than one qualification - just pick one and follow through with it.

As for what to expect working for any of the Big 4...well, don't expect to have any social life. Or any life whatsoever. Be prepared never to have your leave approved, or, if it's approved, to be canceled last minute. Expect to work ridiculously long hours. Expect to have difficult clients, difficult seniors/managers/partners. The burnout rate is quite high.

You might want to consider a career in Advisory. Go for your professional qualification, get sponsored, complete the working experience requirement so you can be fully certified, then apply to transfer to one of the Advisory divisions - Corporate Governance, Risk and IT Advisory, etc. Heck, even the people who transferred to Tax were much happier than the ones who stayed in Audit.