People are saying that Government announced it as we are very tight on stock

Maybe. According to the Govt, this decision has been taken as per the studies conducted in UK and on the recommendation of an "expert panel" It has to be a cover up for the vaccine shortage. SII also stated that they will able to ramp up the production by Aug, so things line up.

In the UK - all vaccines no matter which brand, are given 12 weeks apart including AZ & Pfizer. My boss got Pfizer and confirmed it is the same for everyone. It makes complete sense. AZ has 78% efficacy after the first shot — if you spread it out, more people can get 1st shot which is important.

Canada is also doing 3months+ for all vaccines