According to this web page...
https://www.britishgas.biz/Boiler_Breakdown/HomeCareTermsAndConditions.aspx Quote:A. what is included
1. central heating
<snip>
All necessary PowerFlush work after your initial PowerFlush (which we charge you for). See the 'PowerFlush' section in this leaflet
and that says...
Quote:Powerflush
PowerFlush is a way of removing sludge and other waste from central-heating systems. We may also suggest you correct any design faults which might cause the problem to return. This work can increase the life of your system, and improve efficiency.
When a repair is needed due to sludge (for example, damage to the pump, valves or radiators), we will carry out this job, at no extra cost, as long as we have not already told you that you need a PowerFlush or similar procedure. Our engineer will also tell you what other work is needed to avoid further problems, or offer you a different British Gas HomeCare option.
If we recommend that you carry out a PowerFlush, we will charge you to carry out this work. Once it is finished, there will be no charge for any future PowerFlush work that may be needed as long as you keep a continuous HomeCare 100, 200, 300 or 400 agreement with us at that property and as long as we carry out any work to correct design faults.
PowerFlush is a trademark."
..so I read that as ... if you had one done when you joined the homecare plan then all future Powerflush work should be free.
I note that corrosion inhibitor isn't specifically mentioned but in my view that's essential maintenance like an oil change in a car.