I live within a mile of the Washingborough control and have local knowledge of Sleaford/Lincoln area.
One option would be when going southwards to go straight over at the traffic lights in Branston to Silver Street instead of turning left on to the B1188 - Sleaford Road. Continue on to Mere Road and then via the unnamed minor road to turn left on to Bloxholme Lane. This rejoins the B1188 just north of Scopwick and keeps you off the B1188 for about 10km - but adds about 1km to the distance.
There is no reason why this couldn't be used as the route north as well but the B1188 is not so bad at the weekend.
Unfortunately there seems no obvious way around Ruskington and the A153 unless there is a major route change.
As for avoiding Lincoln town centre -
Instead of turning off the A1500 on to the B1398 into Lincoln continue on the A1500 to go straight over the A15 and A46 via lanes to Scothern. In Scothern turn right to Sudbrooke. Turn right on to the A158 (busy at peak times) for 500m.
(There is a bridleway to Reepham opposite this junction but I'm unsure of it's condition, as it's been a long time since I've used it but it may be an option on a dry day at least).
From Reepham continue to Fiskerton turning left at the junction towards Bardney.
At the eastern end of Fiskerton is an unnamed lane down towards the River Witham. The footbridge here was upgraded some years ago with the addition of long sloping ramps either side with the specific purpose to allow cyclists to cross the bridge without getting off their bikes. This allows you to cross over to Five Mile Lane and on to Washingborough.
Five Mile BridgeThis route was used on an Audax event that used to be run from Clowne some years ago. Taking this route north of Lincoln adds about 8km to the route .
(Those in the know during LEL could have used this bridge to take a short cut to Wragby via Stainfield and Apley avoiding Bardney altogether and saving about 3km.)
As said before, it would be a shame to see the route change drastically and lose the excellent control facilities at Washingborough.
Obviously the fact I live practically on route and if I ever fancy the LEL challenge I'd have the luxury of my own
bed and shower etc has a lot to do with it as well