Hi Louise - if HMS/EDS significantly affects your ability to do day-to-day things, that means it is legally recognised as a disability, and you are protected from discrimination at work (and elsewhere) under the Equality Act 2010. It can be strange at first to accept that you are disabled, but it is really important that you let your manager now formally, and ideally provide medical evidence such as the hospital reports from UCHL. Your workplace then have
to take it seriously and look into making reasonable adjustments for you, eg equipment to help you do your job, sharing/changing your duties, considering reducing your hours, etc. If they do not do this or in any way imply that you are making this up or being lazy, this means you can potentially take them to court for disability discrimination. It doesn't protect you completely from being fired for having too much time off, but it makes it a lot harder for your employer to do this, as they need to show that they tried everything they can (within reason) to keep you in the job.
If you still have problems after officially declaring your disability, Citizens' Advice Bureaux are usually quite good at giving advice on these matters, and also the Equality and Human Rights Commission helpline
is really fantastic. I have used them quite a few times in recent years and they are so helpful!
Hope things get better for you soon!