Sorry to hear of the issue.
We can't speak to anyone for you i'm afraid as we are not a law firm. We are an internet forum and offer online Q&A information and guidance from a legal standpoint.
As for the matter being time-barred, a complaint by the mortgage provider would need to be referred to the Ombudsman within 6 months of the decision (https://www.financial-ombudsman.org.uk/consumers/expect/time-limits). Otherwise I would agree, access to the Ombudsman is time-barred.
However, you may be within time to pursue a claim for a mis-sold mortgage. The limitation period is either 6 years from the date fo the cause of action or 3 years from you date of knowledge. If you have missed the limitation period, you could still pursue it but if the bank raises limitation in their defence after being sued, you would have to apply to the court to extend the limitation period and it will only agree if there is a very good reason to do so.
For a claim, his would need to be through the courts unless you can agree terms wit the bank - which is not possible if they are only offering a token amount of £200.
They will know you have no lawyer hence the very low offer they have made to you.
I would recommend a law firm is instructed - preferably on a no win no fee agreement, so that you pay nothing up front.
Here are a few firms you could try :