Getting an ITIN in the UK as a UK citizen

Hi,

I'm trying to obtain an ITIN from the IRS whilst being based in the UK (UK citizen). The last related thread posted on this forum is from 2015 and is now locked.

Two main questions:
1) How does one obtain a Certified Copy of a UK Passport from HMPO? The IRS requests a 'Certified Copy by the Issuing Agency' in order to recognise it as valid ID. I believe UK consulates abroad do this for a fee, but I haven't found any information for doing this in the UK.

2) There are 'Advisors' that do this service for a fee, with wild ranges in amounts charged (£££). I already have a pretty trustworthy DIY guide to filling out the form, and as far as I can tell the only real value-add service these companies provide is the certification of the ID (see point #1) which I might be able to do myself. Does anyone have any experience using a 3rd party, is there any other value-add and how much is reasonable in terms of cost?

Any input would be greatly appreciated.

Thanks.