No, quite a few sections of the midland mainline were upgraded to 125mph as of December 2013, with foot crossings replaced by footbridges. Elstree tunnel to Wellingborough (roughly) is the most significant chunk albeit with some slower PSRs at places like St Albans stn and Ampthill Tunnel.
Ampthill Tunnel is 125mph, the main section (19 miles) of 125mph is Leagrave to Sharnbrook Junction (north of Bedford). However there is a slow down to 110mph in the down direction through Platform 4 at Bedford (and at the moment a TSR on the up through for 110mph). South of St Albans 125mph is Elstree to just south of St Albans (6 miles). North of Sharnbrook 125mph is only on the down line of which there is a short 4 miles section before Wellingborough and then one more section also on the down line only north of Loughborough for 3 miles. The up and the down line do vary quite a bit in some areas.
Have a look on the sectional appendix 'London North Eastern', download link on the right here: http://www.networkrail.co.uk/aspx/10563.aspx
Pages 407 to 434 and 440 show the speed increases, generally its the HST speeds