If India was to substitute British parts in Tejas and go ahead with the Argentinian sale how would that be perceived by UK? Enough reason to refuse future component sales to HAL? Also, will RR be forced to withdraw the next-gen jet engine co-development offer?
Simple answer is: I have no idea.
Having said that I think
(a) UK respects the wishes of inhabitants of Falkland Islands not to become Argentinian
(b) UK wants to minimise risk of Argentine military action against Falkland Islands
As said, how far UK would go to prevent arms supplies to Argentina I do not know. I suspect though that UK does not have CAATSA pretensions (countries supplying Argentina with arms are somehow sanctioned).
If Tejas were all-Indian, UK might protest about Argentina buying it but that would be as far as things would go, I think. But as to Tejas Mk1A, I don't think that Argentina would select it on the basis of HAL substituting non-British components. If all info about aircraft offered needs to be supplied by end 2021, there is not enough time for HAL to do that, is there? Is HAL going to line up different ejection seat, radome, refuelling probe suppliers in the next 2 months with pricing and delivery known? I don't think so.