Help needed! ICT visa and garden leave

[Wanderer87]

Hello all
I have researched extensively and can not find an answer to this.
I am in the UK on an ICT visa, but looking to switch to Tier 2 general, through the ‘high earner’ exemption, without a cool off period. My old employer was unable to up my salary sufficiently to do so - but I have found another sponsor (a competitor) who have made me an offer and are happy to change my visa.
I submitted my notice, and my old employer has put me on a 6 months garden leave and told me they don’t allow me to change visa or start with a new sponsor before that expires (May 2020).
I think this is seriously detrimental to my immigration position (I am looking for ILR) plus subjects me to the risk of the legislation changes in March / April.
Is garden leave / notice enforceable on ICT? If not on what basis? If I just go and ask the new employer to sponsor me, my work permit with the old employer will indeed expire but they would
Still try to sue me for breach of contract potentially.
Please help if you had this issue before, or have any advice.