The quick answer is ‘no’ for two main reasons.
The first is that few investors invest all their money into equities. You need to have a lot of intestinal fortitude and patience to do this. It is more likely your portfolio will be a mix of equities and higher-quality bonds.
The second is that whatever you have invested in equities really should be spread across a wide range of global equity markets. This reduces the impact of the UK market performing poorly.
So, if over a year the FTSE 100 is down say 15%, the same should not be true for your portfolio. Equally, if the FTSE 100 does really well you may find your portfolio lags that return. It is important to remember you do not have all your money in equities and what you do is spread across the world. This should give you the best chance of success over the long term.