Hi I can offer clarification. The need for it to go to external air IS in fact from the private rented regulations. Not some mystical HAP standards. Thr wording from The Housing (Standards for Rented Houses) Regulations 2017 is:
Suitable facilities for the effective and safe removal of fumes TO THE EXTERNAL AIR by means of a cooker hood or extractor fan.
There is only one set of regulations regarding housing standards. The crap that people spout about special rules for HAP tenants is misleading, wrong and possibly harmful. The issue is that a HAP property has to be inspected within a certain time frame and is therefore prioritised.
With current resources and the amount of HAP coming online, it makes it less and less likely that a non HAP property will be inspected.