Read Wade, or de Smith, or at the least Sumption's recent introduction to the Bill of Rights. Read the GCHQ case and Miller 2.
Approach the Constitution through history. 1649 is important, but 1688 is more important.
Next stop: 1832.
Then 1867, 1884, 1918, 1928, 1972, 2018.
Bagehot is out of date, but still worth a read.
I am sorry to be blunt, but if you find the law of the Constitution difficult, you may be in for a shock when you get to Equity and Trusts.