The full licence is here: https://assets.publishing.service.gov.uk/media/673b0deb49ce28002166a927…

Summary:
Permitted payments are (i) Any payments by any Person that are required (including, but
not limited to, payments for the provision of ground services or
airport services) to enable Iran Air:
• to overfly the United Kingdom; or
• to make a Stop for Non-Traffic purposes (a landing for any purpose other than taking on or discharging
passengers, cargo or mail.) in the United Kingdom
in accordance with Article 5 of the Chicago Convention (in
respect of non-scheduled international air services) and section
1 of Article 1 of the IASTA (in respect of Scheduled international
air services).
(ii) Any payments made by Iran Air for contractual obligations
that arose prior to Iran Air’s designation in respect of ground
services or airport services in the UK (including the refund of
tickets for cancelled flights to/from the UK), provided that no
payments are made to another designated person (as defined
in paragraph 1 of Schedule 5 of the Regulations), whether
directly or indirectly.

Under this licence, subject to the conditions 5-8 below:
4.1. A Person may make Permitted Payments in respect of Iran Air.
4.2. A Relevant UK Institution may process payments made in accordance with paragraphs 4.1

Transactions must be reported within one month.