If you have a EU passport you are anyway still allowed to freely enter the UK.
If you are transiting through London on a true connection (ie. both flights on one ticket) at the same airport you do not need to enter the UK. You will make an airside transit in stead.
According to the UK government website, nothing much will change for you as a citizen until at least 2021.
That means you will not require a passport for traveling to the UK after Brexit if you already have a valid EU Identification Card.
What you’ll need to enter the UK If you’re an EU, EEA or Swiss citizen
What you’ll need to enter the UK will ...