No. Your parents will need to get a visa before they get to the airport in Bombay. Without one, they won't be allowed to board the flight to the UK.
You can check if you need a visa or not for the UK with the Gov.UK Visa Check tool. For your parents case the answer is they need a visa for transit, even a Direct Airside Transit as they're doing. This is irrespective of the length of the transit, a visa is always required in this situation.
If your parents had a Category D visa for Denmark, then they'd be allowed under the exemptions to Transit With Out Visa (TWOV). A Category C schengen visa isn't enough for that though, so they still need a visa.
(As long as they're on one ticket, your parents are fine with only a Direct Airside Transit visa, as their luggage can then be checked all the way through to Copenhagen, and there are Flight Connections buses which'll take them between T4 and T5 airside without passing through immigration. However, if they had two tickets and would need to collect and re-check baggage, then they'd need a landside transit visa)