Tone arm counter weight

Consider using the Rega STOCK, with the best phono stage you can afford.
If you're not, how can you really evaluate the Rega?

An aftermarket counterweight, is just one of many Rega tweaks that will drain you wallet fast. I had a P5 years ago with many tweaks. The  P6 is a nicer table, with the recent changes Rega made.
At the time, the plastic subplatter replaced with an aluminum unit was the only worthwhile tweak. The counterweight was a waste of money. Just my humble opinion.

Caveat emptor with aftermarket tweaks. Cart and phono stage is were hard earned cash is invested with real sonic improvements. Again, my opinion.

Other than that, go up the Rega food chain or try a different brand table in the future.

http://www.expressimoaudio.com/rega301.html
I've used one on a Jelco 750L and found it to improve focus.

I use J.A Mitchell TecnoWeight on my RP6. I have used the same weight on 2 of my previous TTs. Can’t go wrong with it.

Groovetracer.

I also use the TecnoWeight. Any of the underslung counterweights should be an improvement over stock.