Enforceability of restrictive covenant by TCS overseas(UK) based on onsite agreement signed in India

I worked for TCS for client A, in the UK. I recently moved from client A (bank) to client B (retail) and during the same period, applied for a job for a permanent position at client A, having no direct relations with Client A at the time of the offer. Client A TCS manager/TCS HR came to know somehow and sent me a letter stating that I'm in violation of a non-compete clause and asking me to confirm that I'll be rejecting the offer. They gave a window of 7 days to respond failing which, the notice stated that they would 'take action'. The don't have any details of the position I'm joining at, when I'm joining, etc. and the notice per se, is based on hearsay. How enforceable is this in a court in the UK or in India? Also, before 'hearing' about me taking up Client A's offer, they had already confirmed my last working day in TCS and had allowed me to resign at my depute location (UK) which they are now claiming is in contradiction to TCS rules but I already have it in writing that they have accepted my resignation and have confirmed they're happy for me to resign whilst in the UK having given me a 'last date'. Q. What legal action can they take against me if I don't respond to this letter in 7 days? Q. What other action can the company take -i.e. forcing me to return to India to serve the notice period or possibly, termination? Q. Can they take any action after I've served my notice period and joined Client A as a permanent employee?