You don't mention which insurer you're with, but all three of the insurers have a definition of a pre existing conditon in their rules. It's usually defined as a condition that was present before joining, but not necessarily diagnosed. Have a look at your policy documentation to see what the definition in your case is.
In most cases, the determination of pre-existing is made by your consultant in the relevant section of the claim form. You should clarify with your insurer how the determination was made in your case and ask for a copy of the relevant documentation. If it was a case that your consultant stated that it was a pre existing conditon, then you should discuss this with him/her as it may have been a mistake.
If it is a mistake, also ask your insurer how to go about fixing that. It may be a case that they'll request a report from another doctor or consultant or they may ask to see other relevant medical records.
However, there is the possibility that it is not a mistake and that the consultant believes your condition developed on a date prior to you starting health insurance. If this is the case, then your insurer assessed the claim on that basis, hence the rejection. However, if I were in this position I'd strongly query the consultant as to how he/she made that determination (politely of course). If I'm going to end up with large medical bills, I'd want to be certain of the consultant's certainty in the matter.
If the consultant says it is pre existing and you are certain that it is not, then check with your insurer if there are any other avenues. I don't think there are, but it can't hurt to ask.
With regards to the 7 month delay, that isn't uncommon but it's not an everyday occurence. In most cases that I've seen, the insurer requested more information from one person or another and it took that long to get a reply. However, do quiz your insurer on that as well. If they say they requested additional information, ask them who they contacted and how often.
Also, before you were admitted to hospital, did you confirm your cover with your insurer? You're not obliged to, but if you did and they told you at that stage that it would be covered without mentioning anything about the pre existing waiting period, then it weakens their grounds for rejecting the claim and you should refer back to it.
If you wish to refer the matter to the Financial Ombudsman, you need to complete the insurer's internal complaints procedure first, by asking for a Final Response from your insurer. However, if any of my advice above brings new information to light, engage with your insurer to make sure they've had every opportunity to re-assess the claim before referring the matter to the Ombudsman. If nothing else, it makes your insurer look bad if you can show you gave them new information but they did nothing with it.
Hope that helps!