To keep things simple I am assuming that these trunks are used to dial full UK numbers (you will need to adjust the dials rules for anything else)

The trunks can be set up simply from the FreePBX web interface as follows:

IAX:

Dail Rules: 44+0|XXXXXXXXXX

Peer Details:
host=iax5.voiptalk.org
qualify=yes
secret=<PASSWORD>
type=peer
username=<USERID e.g. 84472345>

User Context: <PHONE NUMBER>

User Details:
context=from-trunk
type=friend
username=<PHONE NUMBER>

SIP: