In almost all instances, solar PV panels are a far better choice than a solar thermal system. They are simply more versatile, producing electricity which can also be used for heating instead of just heating water.
As solar panels become cheaper and more efficient, the advantage of choosing Solar PV will only grow.
On top of this, you can also maximise your saving by installing a home battery system to store electricity from the best days, or a power diverter to send electricity to the electric immersion, also giving you hot water.
A solar thermal system might be more advantageous if there is not enough room on the roof of your property to install many solar panels. As they tend to be smaller, you could get your water heated in the same space that would not hold many solar panels.