[buro9]

For company formation, do it yourself.

It's a trivially small fee and can be done online with Companies House in a very short amount of time and they will auto-notify HMRC in the same go.

For mail forwarding and registered office address... I honestly now think you should only consider one of three options:

1) Your work address if it is a sole-use mailbox and you know you will be there for more than a year.

2) Your accountants address (with their consent).

3) Your home address.

I would not now use a third party for the registered address, the risks are too high. There is no junk or spam mail sent to this address, it really is just actionable and important government communication from Companies House, HMRC, etc.

[jsingleton]

I'd be hesitant to use a home address as it goes on public record. If you rent then it can be against the terms of the tenancy contract to register a business at the address. Also you might move and someone else could get your post and potentially do bad things. Are these reasonable concerns?

[buro9]

If you're in short-term rental accommodation (1 year or less) or if you share a mailbox... then yes, these are concerns.

If you fail to respond to HMRC mail, you can be fined. i.e. missing reminders about a tax return, or PAYE info.

Both Companies House and HMRC will send authorisation codes to the registered address, and you can use these to change shareholdings, dissolve a company, make declarations that are untrue, etc.

You definitely need to trust the registered address a lot, and that trust needs to be stable.