Every now and again a question crops up to salivate over - this is one.
Grf has given you the best link to explain Constructive Dismissal - read it all and come back and ask questions if there is anything you don't understand.
As I understand it, you are currently suspended pending an investigation for your comments on Facebook - at least reading the sequence of events you portray above, that seems to be the conclusion. That means that you are being paid in full, so there is absolutely no point in you taking any action to change that situation until the employer decides what to do next.
Sorry to say it, but writing to the company MD whining about this and that whilst the suspension was being investigated was stupid, but it is done now.
The point is that, having suspended you, the onus is now on the company to investigate the allegations that led them to suspend you, then make a decision. At THAT point, and only at that point, is the time to make your next move. Fair enough, learn all you can about the law, and plan what you WILL do when the time comes, but it is absolutely for the employer to decide what action to take, since they are the ones who have suspended you. There is no point in terminating the contract yourself - which may actually be what your employer is hoping you will do.
Please tell us what steps your employer has taken with you to investigate so far. This ought to have involved internal investigation by a manager and should have included interviewing you as part of the investigatory procedure. You have clearly made it harder for them by acquiring Fit Notes whereby your doctor agrees that you are unfit to work at present. That is fine, and is your perogative to do that. However it means that the employer now has to find a way of dealing with the underlying cause that you claim caused the sickness before they can deal with any disciplinary issue - so you have probably held up the investigation and they are scratching their heads to work out the best way around this. That is quite tricky for them and they may look for signals from you that you would be willing to agree to part company from them if a payment is made to you (if you are convinced that is actually what you want). The thing is, they cannot say 'look if we give you X will you leave?' because that initself can be construed as persuading you leave and hence DEFINITELY laying themselves open to a claim for constructive dismissal.
Do not go writing to the employer at this time.
Do not go posting about your circumstances on Facebook or anywhere else again where you can be identified.
But please tell in an anonymous way how far the investigation has got - or have you just had no contact with them?