Pompey Academy were beaten 3-0 at Oxford in their final Youth Alliance League fixture of the season.
But finishing in the top half of the table means that they have now qualified for the top tier of the Merit League.
And Goals from Gatlin O’Donkor, Josh Anifowose and George Frankin ensured that they will be joined by United.
Professional development phase coach Liam Daish said: “The conditions were extremely difficult and it felt like we were in the most exposed area in the UK with all the wind and rain.
“We struggled to find and patterns of play, but were slightly unfortunate to go in 1-0 down at the break.
“The wind was against us in the second half and we just couldn’t get in behind the Oxford defence to cause them any problems.
“They made good use of it, however, to score two long-range goals and I felt, overall, they played the conditions better than we did.
"We’ve qualified for Merit League One and now we’ll look to keep improving and finish the season as strongly as possible."
“It was never going to be the kind of game when you look to play out from the back and it was incredibly tough for both teams.
“Harvey Hughes and Dan Gifford saw their match for Bognor called off, so travelled up and we were able to give them some minutes after the break.
“We’ve qualified for Merit League One and now we’ll look to keep improving and finish the season as strongly as possible.
“There’s been around a 50/50 split between U18s and U16s in the past few weeks and the younger lads will hopefully improve because of that.
“It’s an excellent experience for them and gives an idea of the levels that will be required moving forward.
“So we’re looking to see the lads develop – as individuals and as a team – and there’s certainly a lot still to play for.
“We’ve also got a trip to Farnborough in the Hampshire Cup on Wednesday. They’re flying high and it’s a difficult match, but hopefully we can cause a few problems.”
Pompey: Steward; J.Fox, Kaba, Dockerill (Hughes 46), Laidlaw; Bosaka (Aston 63), Trialist, Simpson; Kamavuako (Gifford 46), Hurst, Howell