Suggesting that nothing would need to be cut to fund it is also flawed. This year the police received £8.6bn from government, next year they are getting £8.3bn, assuming all the additional counter terrorism money goes to the police leaves them with £8.4bn. They already have to cut £200m of costs from their budgets, if they buy tasers they would need to find a further £20m+.
yes things which cost money, cost money and have to come from budgets

your previous post suggested that they'd have to cut officer numbers - I'm pointing out that there exist other budgets
if x is required for 'terrorism' and can be sourced from an additional budget set aside for 'terrorism' then the impact of purchasing x only affects other Y, Z etc.. items which would have otherwise come from that terrorism budget