Go with Raleigh! We did bits of Turtle conservation and English teaching out there anyway. We had a group of Costa Rican's with us who wanted to learn English and we wanted to improve our Spanish, so we spoke Spanglish most of the time
Then at the beach we went on Turtle patrol with the police- and their MASSIVE guns- most nights which was great fun.
When you get your stuff through from Raleigh they send you a massive info pack with general advice for the country and expedition. It includes a kit list- if you want any advice on that I'd be happy to tell you where I got all my things from as it differed slightly from what they sent me- inoculation list (I had to have 6/7 different injections for CR, but it'd be the same anywhere and now I'm all set for the next 10 years!) and a fundraising booklet. This basically includes loads of ideas for raising money, the best way to go about it, what grants and busarys you could apply for from various organisations and how to contact companies for sponsorship- and which companies not to contact. Oh and they send you a lovely tshirt to wear on all your fundraising outings! It was all really helpful although in the end, I only did a bit of fundraising before I went because I needed to fit it around my AS exams, meaning my mum paid the rest for me and I did the fundraising once I got home then paid her back
Umm selection wise, mine was different as it was a joint expedition with a charity and Raleigh, meaning I had already been approached by the charity- RSPB- to see if I was interested in taking part as I do a lot of volunteer conservation work in the UK for them. Obviously I said yes, so we skipped the Raleigh selection process as we had already been 'selected' and vetted by the RSPB anyway.
I assume the physical aspect is just to see how you will cope, as it is very demanding with the climatic conditions. For example, we were walking 30km a day at some stages through really hard parts of the rainforest and beach in stupid temperatures plus the humidity. You have to carry all your kit so you need to be physically fit in order for it to be safe for you to go. We did activities such as white water rafting where it was crucial that you could swim (there is a swimming assessment) as otherwise your life, and others, could be in danger. The physical thing will be to make sure you'd be able to hold your weight within the team, as everyone has to muck in. Although obviously everyone has days where they are struggling which is fine, you can't have really unfit people in the group, as they wouldn't be able to survive. Just get your stamina levels up before you go- I'm a swimmer so my fitness is very high anyway- but if you just go to the gym, for a run or go swimming several times a week before you get out there you'll be fine!
From my own experience, and the others who were on my trip, Raleigh were really great to go with. We had loads of problems- such as British Airways deciding to go on strike, leaving us stranded at Heathrow, meaning we then had to wait over night in Miami Airport for the next available connecting flights- and the UK staff sorted everything out for us at the last minute.
They keep phoning me up asking me if I'm coming again this year, I'd love to but I don't have the time or money... I'm definatly going somewhere with them again though.