It's important to note that it is not illegal to travel from England to India (or indeed anywhere else since last Monday).
The FCDO provides advice for each country (and sometimes areas within a country). Currently it advises against all travel to a few areas of India, notably the border areas with Pakistan and Bangladesh, but a few more besides. As well as this it advises against all but essential travel to the rest of India.
But none of this makes it illegal to travel there. It is not illegal to travel to any destination from England at present. The only difference between the Red, Amber and Green destinations is what you are required to do when you return. What would be interesting would be to find out how many of the many thousands of people who have arrived here from India have booked and remained in approved hotel quarantine accommodation - the legal requirement as bobbin has usefully explained.
If the government wanted to restrict people from travelling to various destinations abroad it could have continued with the legislation that was in place from December to May. This restricted all but essential travel for which permission was required before the journey was made. Instead it produced advice and legislation which is conflicting.
People accusing others of being reckless by breaking the rules on travel to Amber or Red destinations are being wrongly critical. There are no rules restricting such travel. But it has not been unheard of for the government to conflate its "advice" with legislation over the past year.