After a break in, my misses mate told the copper that her fella was going to go downstairs and confront them with a bat.

The copper told her had they came upstairs and he struck them he would most likely have been on, but had he gone downstair and done it he would have been in trouble as he would have been actively seeking them out.

Guess this could be the same, if you actively go out to takle them rather than call the police.

But by the same token, if you heard a noise, disturbed them and got startled and grabbed something to hand?

Think that is the key, you would have to prove you did not go after then with a weapon, but on being attacked you grabbed what was to hand to defend yourself.