would it be possible to walk me through the standard sort of process that I would need to go through after I get a rejection from the IAS?
Once the IAS reject, it's up to Excel to take action. What will happen first is that you will get a flurry of letters from debt collectors - these are written to sound scary, but these debt collectors are ultimately powerless, and can be ignored.
If Excel wish to take the matter to court, they (or more accurately their solicitors) will likely issue a Letter of Claim. If you get such a letter, or you get a claim form from the county court, come back here promptly for advice.
In particular, I would like to avoid having any impact on my credit score as while I have the time to make it not worth their while to shake me down for money, I would also like to avoid taking unnecessary financial costs if necessary (especially as I plan to take out a mortgage at some time in the next few years).
Providing you keep on top of the process, there is essentially no risk of your credit score being impacted. Even if you were to lose in court (which doesn't happen very often), then provided you paid up promptly, the CCJ is removed from the public record, and there is nothing to impact your credit score.
The main thing to be mindful of, is if you move house before the matter is concluded, write to Excel to inform them of your new address for service.
Happy to receive a PM/email if needed.
Where possible we try to keep correspondence on the thread, rather than via private message. This allows multiple people to offer assistance, and means that others who visit the forum can read your thread and gain information that may help them in their own case.