Also, ask yourself if this is going to be a huge issue. Women who were unsure about wanting to have kids and then suddenly wake up one morning thinking "I've gotta have kids!" tend to be in the minority. I've talked to several women who were unsure and none of them ever had that moment. Whether they had kids or not, they all said it would have been okay with them either way. If this is you, then the resentful day you fear might not even happen.
And if the guy actually went through surgery to avoid having any more kids, that's a really good indicator that he's made his decision. And it's been my experience that most guys are nervous enough about getting snipped in the first place to even consider having another surgery done "down there".
If you absolutely know that you want children then you have to finish it unless he is completely committed to vasectomy reversal and IVF etc. The baby hunger grows, it does not lessen with time and you will end up resenting him. There is nothing wrong with being childless but if you believe you want children, you shouldn't give up on that possibility because the drive to be a mother grows as you get older. I think it is one of those fundamentals of life that you cannot compromise on - both people in a relationship need to want the same outcome, whatever it may be.
You need to talk to him about your concerns. He can always have his vasectomy reversed, it may just be that he doesn't want to.
Sit down and tell him that you do love him and want to be with him. But that one day you may want to have children of your own with him.
talk talk talk!! There's no point trying to make these decisions on your own!
A vasectomy is reversable, though I am not sure if you get it on the NHS; anyhow, speak to him about your feelings. Maybe he would consider reversing it and having a child with you if it would be really impotant to you. People can change their minds about having children if they feel they are with the right partner.