Your estimate of 6 hours could be very optimistic. Lots of M-ways have roadworks going on, accidents that slow you down, etc. In my book even that is too long to do in a single day though I know others will disagree. Many M-ways have Travel Lodges or similar, some very cheap if not near prime areas. You will likely be more stressed than you think and that is not good for a driver. I would suggest an overnight at about 60% of the distance from home, making a shorter day ahead.
A satnav is virtually essential. Either on your phone (which will be bang up to date) or a dedicated machine (which might not be). We have both and often resort to the mobile phone. I wouldn't bother about being able to SEE the route on-screen, but listen to the instructions carefully. As others have said, if you go wrong it will get you back on track.
Some junctions are a doddle, others need more care. Try to track the route before you go and take extra care at these - particularly where changing from one M-way to another.
As others have said, stay in the inside lane where possible.
We regularly travel from Manchester to Bristol and find the M5 has much less traffic than the M6 towards Birmingham, but actually leaving the M6 for the M5 is fine - just don't get on the M6 Toll Road as it doesn't connect with the M5.
I would also factor in a service station every 2 hours. You generally need 40 mins for parking and toilet break plus buying a sandwich, more if you sit in to eat.
Huge lorries can be disconcerting. They have a timetable to stick to so will overtake you. They then pull in front of you (in your "safe space" once they have got past you. Allow for that and ease off the speed - you'll soon speed up again.
Lots of drivers only do about 60 mph these days - the speed merchants are in the outer lane.
I don't agree that A-roads are easier. All those junctions, traffic lights, roundabouts, single lanes, parked cars, side roads where cars can whip out unexpectedly. Give me the Motorway any day.
Nor do agree with music en-route. Too distracting.
Plan it all carefully. Be prepared for occasionally getting in the wrong lane - the sat nav will sort it for you once it realises. Don't panic at all, and you will be surprised how competent you really are.
A coach will take twice as long. Airports are not always convenient. Trains possibly worth a look. Connections aren't always convenient.
Good luck.