I would imagine that all existing estimates that were presented to you as of when you signed up to your ISP would be valid, including the handback. I don't see how activating a second line at the property would invalidate your handback speed, but I would say that those speeds on the DSL checker, including the handback speed, are always adjustmented based on data collected anyway. I could be wrong, but it would be safe to assume that this is correct unless anyone here has experienced the contrary.
£50 is pretty cheap for a brand new line being fitted. Some providers may absorb or reduce the cost of a new line installation if you take out a lenghty contract with them, and it looks like Plusnet are doing just that. CIX (
https://cix.uk) seem to have free line installation when you take line rental with them but of course this comes with a 12 month contract. Other than that, some providers may charge in excess of £100 for a physical line to be installed (I know
https://pulse8broadband.co.uk/ charge £129.00 for that, but the bonus of this provider is that it is only a 1 month contract with no cease fees).
I would recommend perhaps looking at a different provider using a different backhaul if you are looking for realibility (since Plusnet use BTW, perhaps look at a provider using TTB).
DISCLAIMER: Just because I have mentioned providers in this post, it does not imply that I recommend or am affiliated with them in any way, shape or form.
[Moderator edited to fix a typo.]