The first train from Loughborough to St Pancras is 5:14am arrive at 6:54am, the second one is 5:54am arrive at 7:36am. It takes about one hour 30 minutes from St. Pancras to Heathrow. Seems the second train is bit too tight.But you can go to Paddington station to get Heathrow express (total maybe an hour from St. Pancras but cost about £25).
You can get on google search 'London Underground' and 'St. Pancras to Loughborough'. Book ticket earlier will get lot cheaper otherwise, cost you dearly.