The £39bn is not significant at all. It's payable over 60 years according to our esteemed Chancellor.
Only a very small part relating to pensions goes that long. Wiki:
In March 2018, the UK's Office for Budget Responsibility (OBR) published the UK's economic and fiscal outlook including details of the financial settlement which it estimated at £37.1 billion (€41.4 billion).[27][28] The estimated settlement was made up of:
- £16.4bn (€18.5bn) towards the UK’s contribution to the EU budget to 2020 (after offsetting for the UK rebate);
- £18.2bn (€20.2bn) towards outstanding commitments for projects that have been signed off but not yet paid for by 2020 (The Reste à Liquider ("RAL") from successive Multiannual Financial Frameworks) to be paid up to 2028; and
- £2.5bn (€2.7bn) for other financial liabilities, being an estimate for pension liabilities of €9.5bn offset by other assets totalling €6.8bn.[28] The payments towards the pension liabilities are estimated to be made until 2064.[28]