It secured her third England vest in successive years over cross country, and her fourth overall.
The championships were held in freezing conditions with snow blizzards and gale force winds on the magnificent grounds of Temple Newsome House in Leeds, and the superb course was a real tough test with many steep inclines to deal with.
Williams was in the first race off, and she made the most of the firmer conditions and followed her pre-race plan to run the first 1500m aggressively and to stick right at the fore of the race in the hope of a breakaway group forming.
She found herself pushing the pace and battling with four athletes in with a chance of securing the individual bronze.
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Williams continued to push over the closing stages to finish in 13:59 and seal her call to represent England at the SIAB International on the March 24.
She said: “I’m delighted to have qualified for SIAB this year.
“I was so pleased when I crossed the line and knew I had secured a vest.
“I’ve been training hard and was hoping I could do it.
“To get an England vest was my main goal this season and I’m so happy to have have acheived this.”
Williams’ coach Shane Smith added: “Over the past two years Emily has shown amazing consistency on both XC and on track.
“She has shown she is an athlete who always produces on the big occasion.
“We are now looking forward to the International race at Wollaton Park in Nottingham, the course where she made her breakthrough just two years ago.”
Other Harriers performed well at the championship with bottom-year Senior Girl Poppy Carmichael continuing her strong form in the closing stages of the season, finishing a fine 139th over the 4.1km distance in 18:39, her best show for the past four years.
Harriett Jolley came in 251st (20:12) and Amy Robinson ran a solid race for 279th place (20:57) out of 323 finishers.
Bottom-year Intermediate Boy Dylan Bowley ran well to finish in 208th place in 20:14 over the 5.2km course, helping the Northants team to their highest placing on the day in 25th.
The final two Harriers in action were competing in the Senior Boys race over 6.8km with Josh Cannell making his English Schools debut running well to finish in 294th place in 28:55, closely followed by Tom Jawad in 297th (29:36).