I agree that Money supply/Money stock is negatively correlated with short-term interest rates.
Since Great Britain has been using the Sterling before and after Brexit, there is no fundamental change in Monetary Policy caused by Brexit. Your (implied) assumption that Brexit causes a change in interest rates is therefore highly speculative.
Rather, the Bank of England's Monetary Policy Committee decides on interest rates based on (forecasted) growth and inflation rates, and a big number of other factors.
The economic performance (and therefore, interest rates) of the UK in the aftermath of Brexit will depend heavily on the negotiated deals between the EU and the UK.
If the UK economy indeed slows down as a response to Brexit, as many assume, the BoE will most likely keep the interest rates down. However, this is not at all guaranteed to happen.