They might prefer to lease them at a lower rate and not take P&L hit
They could do, but I understand that's not happening at the moment. These forums are full of stories of rolling stock being sent to the scrapyard when they still have 10 or 20 years of useable life left in them. I may be wroing of course, but this is my understanding of why that is.
I would expect that the leasing companies will be driven by future cash flows, rather than by accounting asset value. (Though they may well record an elevated asset values for the 222s, as this would help them to "justify" their high leasing charges, to DfT and to the public.)
But I would still expect the leasing companies to hold out for high leasing charges for old stock like 222s. They know full well that within a year or two, the DfT is going to bow to political pressure and simply instruct a captive ToC to lease the 222s, at whatever price is being asked; or else instruct them to order more new stock instead. Which will be leased, of course. So whichever way DfT jumps, the leascos will win by holding out for higher leasing prices for old stock.
Remember, the economic incentives on leasing companies are not the same as those of the taxpaying and travelling public.