Sophie,
I'm not an HR professional but I am based in the UK and have worked on the periphery of HR for a number of years so I have a little knowledge here but I am not an employment law specialist and so my advice comes with the obvious disclaimer.
First of all, in the UK, discrimination based on gender reassignment is just as illegal as racial or sex discrimination. Therefore, if the issues you have articulated have arisen purely because you are transitioning, your employers have acted unlawfully. The problem, of course, is proving it. There are many instances of discrimination against certain groups occurring every day - groups such as women of childbearing age, women wearing headscarfs, people in their 50s, the disabled to name a few. Of course, as we know from the press, any recruiter that says 'I'm not employing you because you're bound to go and have children' or 'sorry, no headscarfs' very quickly lands themself in hot water. And it's the same for the TG community. The solution, of course, is to set the recruitment parameters such that another candidate will score higher than the candidate from the group to be excluded and good recruiters will retain an audit trail to prove that they went through due and fair process.
In your position, I think there are a couple of things you could consider:
1. Look for a pattern; has the attitude of your management changed since you revealed your intentions - in particular, can you provide specific examples of similar incidences before and after the reveal where the outcomes have been different? If so, you've got stronger prima facie evidence of discrimination; they could, of course, try to say that you have been affected by the hormones but, as half of the population is already on oestrogen, mostly without issue, to my mind that would provide greater evidence of discrimination.
2. Make a friend in HR, if your company has an HR department. HR professionals tend to be obsessive about employment law and a word in the right place may well filter back down to where you need it.
As for raising a grievance, always a difficult call as it doesn't tend to endear you to your organisation but if they did make life intolerable and force you out as a result, I'm sure a tribunal would have a field day.
I hope that this is some help.