If you're fussy about where you want to be based then perhaps the course isn't for you. With any of the CTC partner airlines you can get based anywhere and you take what you're given. For easyjet specifically it is much the same. You can stay flexicrew and not leave the UK or you can wait until you are eligible for a UK contract. You are not forced to work abroad. Barring BA (London) you will not get to choose where you work.
After 3 years your experience will hopefully be a commodity that can take you elsewhere. If you are not offered perm employment you can go and work for someone elsewhere. There will be little difference in salary if you choose your new employer well. The bond gets repaid, this is correct, but you are on a reduced salary in the mean time which means you are effectively on the same salary as someone on a full contract with no outstanding bond, only difference is you are not liable for tax on £12k of your annual salary. You could both take home £36k before tax so leaving for another airline on a full contract with a £36k salary will not really leave you that worse off.