You may want to consider a fixed 10 mag scope. You are likely to get better glass for your money and many, including myself, use 10 mag for HFT. Not too a big objective as well so you have decent depth of field.
Ones to look at sub £200 second hand would be an MTC Viper, EB Sniper or Hawke Sidewinder.
I still have examples of the first 2 and they are very good scopes.
I'm currently using Sightron SIII's for HFT as the glass is fantastic and suits my poor eyesight. They are around £350 second hand so maybe a bit minty to start with.