Hello, my name is Ben and it is my pleasure to assist you with your question today. There is actually no legal right to receive time off in lieu (TOIL) and this is something left at the employer's discretion. It is in effect treated as overtime and as there is also no right to paid overtime, whether you get paid for TOIL owing at the time of termination is something that depends on your contract, a workplace policy, or whether it has been consistently done over time with other employees.The Government simply confirms that the terms regarding TOIL are effectively left to the employee and employer to negotiate:https://www.gov.uk/overtime-your-rights/time-off-and-paid-leaveFurther confirmation of your rights to TOIL and that it is not a guaranteed payment (unless overall you are getting less than the National Minimum Wage) can be found here:http://www.workingrights.co.uk/time-off-lieu-working-additional-hours.htmlSo unless there was some contractual right that entitled you to payment for accrued TOIL, it would be rather difficult to argue that point and a constructive dismissal claim may not actually succeed (such claims are traditionally rather difficult to win anyway).
Of course none of the above would prevent you from discussing this with the employer and negotiating but if they are adamant that the current rules stand, then unless you can show that some of the above apply you will be in a difficult position to press for this without potentially creating conflict
Please let me know if this has answered your query or if you need me to clarify anything else for you in relation to this?