I am working on an assignment to calculate Beta in the CAPM Model through empirical data on the british market and am still unsure which risk-free rate to use.
Since I have a 1 week investment horizon, my first Idea was to use short term UK government bond yields provided by the Bank of England site (https://www.bankofengland.co.uk/statistics/yield-curves) but their data has many gaps/misses completely after 2013 for bonds under 1 year.
So my question is should I then use a bond with a longer maturity, despite my short investment horizon?
Could I alternatively also use other rates provided like Libor or Sonia? I have found Libor rates whith a matching 1 week maturity and also Sonia rates but I am not very familiar with those.
Any advice on the topic would be greatly appreciated