Hi, we are also in the UK. My son was diagnosed 2 years ago aged 14. He was initially started on EEN and Pentasa which helped for a few months. As he continued to have symptoms he was put on Azathioprine which he had to stop after a few weeks due to low lymphocyte count. He then tried 6MP but had same issues as with Aza. In March this year, as he still wasn't in remission, he was given the choice of Infliximab (Remicade) or Humira. He chose Humira as it means fewer hospital visits. Still not in remission so Methotrexate recently added. I think this 'bottom up' approach is quite typical for the UK unless symptoms at diagnosis are really severe.
The NICE guidelines state that biologics should be considered for 'Severe, active Crohn's disease in people aged 6-17 years whose disease has not responded to conventional therapy (including corticosteroids, immunomodulators and primary nutrition therapy), or who are intolerant of or have contraindications to these treatments.'
Good luck!