Why is it still so cold in London even though it's April?

This is normal.  Every few years we get a cold snap in April.  In 2010 I was working on a road scheme in Kent and we were SNOWED OFF for several days in April.  Less than three weeks later it was over 30.  That felt crazy!

In the bigger picture though, later springs are becoming a thing in Britain.  It gets cold later in the year than it used to so all kinds of plants now flower in December that used to either have died back or wouldn't flower until the next Spring, so plants we never used to see together now co-exist.  It's also changed the order in which some spring flowers come.  I knew we were going to have a cold spring because my bluebells are very late this year.

It is normal to get low temperatures in April, and sometimes in May. The UK is situated well North of the Equator.

It is often still cold in May. This present cold snap is coming from the Arctic and bringing snow to parts of the country - but London and the SE will be the warmest.

Because Global warming is a Hoax

Moscow is even 'hotter' than London today (12 April), but the Daily Mail will NEVER splash that across the front page, as it would if the south-east - of England - was slightly warmer than Malaga.

Many places in the world are still cold in the month and it's not unique to London. That's why the queen's official birthday is celebrated in June instead of the actual day she was born, which is April 21. When Charles succeeds her, chances are his birthday won't be celebrated in November. It might be celebrated in September instead.

Because it's London. It doesn't get warm until July. 

Spring weather can shift quite easily in either direction.

It's Spring; weather is variable at this time of year. 

Global warming of course.