This is precisely the operation in which UK SF would excel.
This problem is too large and diffuse for UKSF at this stage. Nobody even knows where the girls are being held now. The best guess is that they are probably dispersed amongst many Boko Haram camps and caves in the Sambisa forest area in the mountains bordering Cameroun. I've flown over the area and it is huge (about four times the size of Wales) and dotted with tiny villages, hunter camps, caves. Some areas are fairly open and accessible, but other areas are thick jungle.
The only way to search it is with aerial reconnaissance. The Nigerian Air Force does have modern photo-reconnaissance assets (ATR42 with EO/IRs, B350 with vertical camera, DA42s with EO/IR) which would be suitable but they were mainly acquired so that corrupt officers could chop back-handers from the contracts. The NAF is far too feckless and lazy to actually use them and maintain them, and they would never be able to analyse and interpret the images anyway.
The rainy season is now underway, so satellite imagery may not be possible and aerial-reconaissance would have to be flown under cloud base which is typically 2000-3000ft agl. The other issue would be where to base aircraft. There is a NAF airforce base at Maiduguri, but it is completely insecure and you would not want to rely on the Nigerian Army to protect the base in the event of a Boko Haram attack (it has already been attacked a few months ago - several aircraft destroyed and many killed). Kano and Makurdi NAF bases are a bit too far. N'Djamena in Chad would be the best logistical choice, or Garoua in Cameroun would be closest - but the Nigerians will be reluctant to allow cross-border flights.
However, the biggest obstacle to any outside intervention will be feckless and venal attitude of Nigerian officials. They will object to any plan that will see them losing an opportunity to chop some money.