If I had an email proving my manager was just waiting for me to quit due to her behaviour

Get her to give you a copy of the email and then file a subject access request with your work - where they have to give you any document (email, report, memo - everything). They have the right to redact other people's names but they will have to give you everything.

I think it sounds like you definitely have something there for constructive dismissal but an SAR should throw up more evidence.

Btw make sure you have copies of all the emails you sent in asking for more work too. Don't email them to yourself, just print them out and take home the copies (or screenshot them and email the screenshots to yourself).

Friend/manager will probably be dismissed if she gives it to you, and I am not certain she wouldn't deserve it either - as a manager one is often privy to stuff that you absolutely should not share.

There is nothing unlawful about a workload decreasing, and there is no entitlement to developing your role. If there isn't enough work they should make you redundant.

The short answer is no. Constructive dismissal is simply not as easy as people think it is. And you can be absolutely certain that your friend will be blind, deaf and dumb the minute you try to. That email won't ever have existed. But before you make any claim you must normally have exhausted your employers internal grievance procedures and also be able to demonstrate that they have committed a breach of contract so fundamental as to justify leaving. Run it through your head - "I resigned because my employer paid me to not do any work". How does that sound? Not very convincing? You'd have an uphill battle, with little evidence - if you have any evidence at all. Even if you get that email, she can easily claim that it was a confidential off the record moment of exasperation, that the annoyance was because you just keep wanting work when there isn't any to give, and really she should have been flagging this as a redundancy for you but didn't want to feel responsible for someone losing their job especially at this time...so letting you quit is a kindness.

Given how very hard it is to win such a claim with copious amounts of evidence, you would have to be past desperate to try it with next to none.

I would want to see the actual email before I did anything. Could your friend be stirring?

That does not sound like grounds for constructive dismissal to me OP.

I have a friend who is also a manager in the same organisation. They have had sight of an email where my manager admits to me being an “annoyance” and she has no doubt I will “quit soon enough”.

That doesn't tally up with your thread title at all?

It doesn't mention your manager's behaviour and you don't have the email?