What can I do now before it gets too deep financially?
The financial depth is £140, the amount on your NtO. Only if you ignore the NtO and any subsequent enforcement documents can it get any larger. If you end up taking them to London Tribunals, the £140 remains the maximum, there are no other costs of an adjudication.
The main point about the Notice to Owner is that you
mustrespond to it, (pay or appeal it). As the discount option has now expired, it is a total no-brainer to take them all the way to London Tribunals, but first you must submit representations again to Hillingdon. They might give way at this stage and cancel, although it is very unlikely. What is more likely is they will re-offer the discount. However, if you are happy to have £140 at risk, you can, after they reject your reps, register an appeal at London Tribunals and present your case to an adjudicator. If you win, you pay nothing, but the council must prepare an evidence pack and pay the LT adjudication fee so it's a PITA for them, so they might not contest the hearing. However, you'll only find out if you decide to go for an adjudication.