Heathrow and Boris Island runways appear to be almost exactly in line, due east-west of one another, I make them 39 miles apart. The mid-point along this line (so when Heathrow is on westerlies this is what it will be like with Boris on easterlies) lies in the Thames between Canary Wharf and the O2, about 19 miles each way.
So if you go to Canary Wharf and look at the height of the overflying Heathrow traffic on westerlies, that is presumably where it will be when Boris Island is on easterlies. It's just where most traffic coming off Lambourne and Bovingdon is turned right onto long final. We live east of this point, further from Heathrow, and this traffic is certainly still prominent here.
The easterly track to Boris appears to descend from there over Woolwich, Bexleyheath, Dartford, and (particularly) Tilbury/Gravesend. However, Boris Island is portrayed as an unrestricted H24 airport, unlike Heathrow currently, with full parallel approaches.
From time to time Heathrow is on Westerlies while London City is on Easterlies, which leads to interesting separation over Waterloo as seen from the ground. The prospect of Boris on Easterlies when City is on Westerlies, however, seems even more of a challenge.