Do not buy a HALion 6 to Absolute 6 crossgrade, as the code will not work - that is intended to upgrade a HALion 6 licence (on eLicenser) to Absolute 6 on Steinberg Licensing.
You can use the other crossgrade offer to Absolute (from a list of Steinberg products) if you have HALion 7, but the UK price is around 70% of the full price of Absolute 6! Steinberg strongly incentivises buying Absolute outright, not crossgrading to it. The crossgrade offers from HALion to Absolute have never been that generous.
I would wait for a sale on Absolute before proceeding - in the past few years, Absolute has gone on sale at somewhere between a third and half off around once a year. Depending on the sale deal, you might be better off buying Absolute outright, then selling your HALion licence once the sale is over. You will probably have some time to wait, though, as the last Absolute sale was at the end of last year, with many lucky purchasers activating Absolute within the grace period for Absolute 6 (so they’ll get an Absolute 6 licence free of charge.