Hey Buddy
I am also attending Heathrow tommorrow (19th). I THINK it is just the initial selection tests i.e, as you say Maths, english etc.
They do usually send you an email every few weeks to let you know in advance what you will be doing on certain dates that you attend if you pass each stage of the process.
This day is scheduled to take between 3-4 and a half hours and I cant see the tests taking that length of time!, so maybe they will add something else for us all to do???.. It's all pretty basic stuff mate, the tests are not designed to catch you out or make you look like you have a slack standard of education. Just that you have a basic understanding.
All the stages usually take around 3-4 months to complete. Initial tests, Interview, group assessment, role paly etc, interview and medical.
May catch you at Heathrow tommorrow, If not then the very best of luck to you.
Regards. Tazzy2040.