Back to school or smaller firm?

Hi all, 

Firstly this is aimed at the U.K ...

Here's where I am:

- GPA 3.4 (2:1) (non-target school)

- 1.5 years as a graduate engineer, then 3.5 years big 4 experience (both consulting and audit work) 

- No masters, no CFA, no ACA, no GRE/GMAT scores etc...

I don't want to do IB

I ultimately want to be something like an ESG investment analyst in AM (fascinated by the crossover of sustainability and finance)..

I have an interviews for a trainee investment manager and investment analyst positions at a mid-market investment management firm. If I get it, is this a good move?

I have had mixed advice about what the best route forward for me is, the options as I see them are:

- Focus on CFA and learning investing, financial modelling etc in my spare time and apply for smaller/boutique investment management firms (what are the prospects here?)

- Focus on GRE/GMAT then then try for a top MSF school (can't afford 30/40K in the UK so was thinking St Gallens or RSM?) then from there get an ER role or similar at a big institution (Fidelity, PIMCO, Blackrock etc.. realistic?)

Which would you choose, what are the pros/cons of each, chances of me getting something without a masters. Risk/job prospects of choosing a smaller firm?

Thanks