Get AAAA ---> Oxbridge, LSE, Warwick, UCL, Imperial ---> Pick up work experience in banks ie. back office roles/spring weeks (network well here; make the right contacts) ----> at uni: do a few extra curriculars i.e. positions of responsibility, student journalism, Model U.N, AIESEC, start up a business and turn a profit (the hardest, but most respected), part time job, finance/investment/entrepreneur society, Traders society etc (just some example, there are better ones I am sure.) -- join it and do something credible and tangible, maintain a 60% minimum average, of course a 1st ALWAYS looks better than hitting the minimum 2.1 ----> 2nd year apply for internships ---> hopefully land an internship with a BB/2nd tier firm ---> convert internship into an offer/otherwise leverage the experience and apply to all other banks which will surely yield an offer in investment banking. Graduate with a 1st/2.1
Note: investment banking is a very broad field, you need to find out which role you think is the best fit for you i.e. sales/trading, research, IBD, private wealth, operations, risk management, finance, technology etc
Just something I threw together based on observation and successful candidates I personally know or on TSR.