Second-rower Martin and loosehead prop Whitcombe both started for England Under-18s in their 41-22 defeat by hosts South Africa while Costelow was among the replacements as Wales were beaten by France.
England led 17-12 at half-time with tries for Tom Roebuck and George Barton who also kicked seven points and led 22-19 when Roebuck added his second but South Africa claimed four scores in the second period to secure victory.
To reflect a change in the England pathway calendar the group in South Africa are predominately under-17s with the tour now marking the start of the under-18 international season.
England coach Steve Bates said: “This has been a really steep learning curve for these players and I think they’ve come out of it with a lot of credit.
"There is lots of talent in this group, but they had not played together at all before we arrived.
“They haven’t played much rugby for three or four months but have worked really hard for each other. We could not have asked any more of them in terms of their effort and commitment, but they were outmuscled today after an hour – before that though we had played some really good stuff.
"We knew this was going to be a challenging tour, we brought the youngest squad in the tournament and I think they have learned a lot in difficult circumstances.”
A dominant French side beat Wales 52-7 on the final day of the Under-18 International Series.
France, who fielded an under-20s development side throughout the series, were always going to be a tough proposition for what is effectively an under-17 Welsh team and went into the game on the back of wins against England and South Africa.
Replacement fly-half Costelow was included among a raft of changes in the second half and played a key part in the Wales score, calling a move from the scrum that saw Joe Roberts hit the ball at a fine angle to claim a try. Costelow also added the conversion.