The M3 is always less busy than the M4.M25 is busiest between 3.30 - 6 pm and 7.30-10 am weekdays and especially hellish on Friday afternoons. If it were me, I would wait until 7pm, or if you can't, just accept that it will be a bit sticky ( a few good tapes and a couple of bottles of cold water might be an idea).....take the M25 to the M3, the M3 to the A303,come off at Andover and follow the signs for Salisbury.

This is the route the RAC recommend

Start at LONDON STANSTED AIRPORT (UK) direction FLIGHT DEPARTURES AND ARRIVALS. 516 yards
472 m 0.0 miles
0.0 km
2.   After 516 yards take the 1st Exit off the Molehill Green Roundabout (Roundabout) onto Terminal Road.
0.7 miles
1.1 km 0.3 miles
0.5 km
3.   After 0.7 miles take the 1st Exit off the Coopers End Roundabout (Roundabout) onto Parsonage Road.
1 miles
1.6 km 1.0 miles
1.5 km
4.   Continue straight onto Parsonage Road. 592 yards
541 m 2.0 miles
3.1 km
5.   Turn right into A120 (Dunmow Road). 729 yards
667 m 2.3 miles
3.7 km
6.   Continue straight into A120 (Dunmow Road). 2.4 miles
3.8 km 2.7 miles
4.3 km
7.   Leave the A120 (Dunmow Road), and join the M11 at the M11J8 junction direction [M11] LONDON. 13.9 miles
22.4 km 5.1 miles
8.1 km
8.   Leave the M11 at junction M11/M25/J6/J27 and straight ahead into M25 direction [M25] WATFORD, HEATHROW AIRPORT via [M4]. 50.5 miles
81.3 km 19.1 miles
30.6 km
M25 Clockwise J24 -J25 (Potters Bar - A10)
This section of road is a known traffic hot spot.
Call 1740* from your mobile for updates.

9.   Leave the M25 at junction M25/J12/M3/J2 and straight ahead into M3 direction SOUTHAMPTON, [M3] BASINGSTOKE. 31.7 miles
51 km 69.9 miles
111.8 km
M25 Anticlockwise J21 -J11 (M1 - A320 Chertsey)
This section of road is a known traffic hot spot.
Call 1740* from your mobile for updates.

10.   Leave the M3 at junction M3/J8 and straight ahead into A303 direction ANDOVER, SALISBURY, [A303] THE SOUTHWEST. 15.8 miles
25.4 km 101.8 miles
162.9 km
M3 Southbound J2-3 (M25 - A322 Bagshot)
This section of road is a known traffic hot spot.
Call 1740* from your mobile for updates.

11.   Continue straight into A343 (Salisbury Road). 858 yards
785 m 117.7 miles
188.3 km
M3 Northbound J3-2 (A322 Bagshot - M25)
This section of road is a known traffic hot spot.
Call 1740* from your mobile for updates.

12.   After 858 yards take the 2nd Exit off the Salisbury Road (A343) (Roundabout) into A343 (Salisbury Road).
517 yards
473 m 118.2 miles
189.1 km
13.   Continue straight into A343 (Salisbury Road). 7.8 miles
12.6 km 118.5 miles
189.5 km
14.   Bear right into A30. 5.9 miles
9.6 km 126.3 miles
202.1 km
15.   You are entering Salisbury.