Big four transactional advisory to investment banking

Apologies for slow response. As you say, CFA L1 may help you jump across to Big 4 advisory from another part of the firm - it demonstrates your interest and gives you a reasonably grounding for the basics.

However, I'd be inclined to suggest you do the CFIs FMVA course instead - it's cheaper than doing the CFA level 1 and gives you far more practical modelling skills. Even if you were just to do the LBO and M&A modelling modules. From my experience, modelling skills are pretty poor within the Big 4, at least at Manager level and below, so this would put you ahead of the pack.

Equally, making sure you fully understand and M&A process (sellside vs buyside) and what your role would involve is really important. Sounds basic but to provide a useful and funny example of where someone failed to do this - a friend of mine interviewed an audit guy looking to transfer into CF - he asked "Can you explain what a data-room is and what it is used for". The candidate thought that a data room was a physical room in which client information was stored. Knowing the basics really well is what gets you across.

Seperately, worth noting that a number of the big banks are now looking for newly qualified ACAs to move directly into analyst positions e.g. JPM. Certainly worth looking at this if you were starting out down the ACA route. I know Roths are also pretty keen on ACAs.