Isn't 120k AS miles worth a boatload more than 500 GBP?
OP is talking about CX Asiamiles not AS Mileage Plan.
That being said I have gotten good usage out of AM on JL and AA the last couple of years, multiple business class redemptions that would have cost way more than £500 for 120k AM. I’ve redeemed a lot of AM, but precisely zero of them flying CX metal.
AM has some sweet spots where the mileage pricing on AA/JL metal is approximately what I would have paid using AA/AS, with the advantage that AA and AS don’t have a lot of mileage transfer entry points (Bilt and Marriott for AA, just Marriott for AS), but CX has a lot of them (AMEX, Bilt, Capital One, Citi, Marriott). They also charge less YQ for USA->LON than AA/BA/AS do on a BA redemption.
But if OP is happy…