There is an explanation in the Preliminary report here:
"This liquidity has been raised from a diversified range of funding sources including: the utilisation of our £65m Revolving Credit Facility; the drawing down of the Bank of England's Covid Corporate Financing Facility ("CCFF") of £199m; the financing of unencumbered owned mid-life aircraft for £102m; the sale of Fowler Welch, our non-core Distribution & Logistics business, for gross proceeds of £99m; plus two oversubscribed equity placings raising gross proceeds of £594m, for which we are grateful to our Shareholders for their support. Post the financial year end on 31 May 2021, Jet2 plc signed a new unsecured £150m term loan maturing in September 2023 with its supportive Banking group. In addition, on 3 June 2021, the Company announced the successful issuance of £387m of senior unsecured Convertible Bonds due in 2026 carrying a coupon of 1.625%, the offering for which was heavily oversubscribed."
So yep, I believe they have the money, however.... £903 Million of it, almost two thirds, is Debt, the credit facility, the CCFF, the loan, the aircraft, and of course the senior unsecured Convertible Bonds.