Handling nightmare/overbearing family

You will find a lot of forums on here about overbearing relatives so you will see some of the same advice. I would suggest you and your partner agree terms of engagement with people about wedding discussions so you are on the same page and will back each other. If you agree to ignore outside influences then you can agree stock phases to use whenever someone makes a comment. The key is showing a united front with your partner, especially if the main issues could come from their side. You need to know your partner will back you if you say anything to your MIL, or say it themselves if you don't feel able to.

The most important thing is that you and your fiancé are of one mind. You describe his mother as an 'ex' EDL member - unless she has openly repudiated her links (which seems unlikely, considering her new man is an 'active' member), she is still likely to hold those views, even if she isn't very vocal about them. Which probably explains her lack of enthusiasm about your engagement. Are you 100% confident that your fiancé is going to support you and stand up for you, even if it means potential estrangement from his mother? You don't want to end up feeling that you have to suppress your culture or to try and 'whitewash' yourself to pacify MILs racist views.

Regarding the wedding planning, I would put the family on an information diet - if they don't know what you are planning, they can't hassle you to change it. You can always present this as 'we're keeping our plans a secret to surprise everyone on the day' to make it more palatable! However, I would make it clear to MIL that there will be a mix of traditions from both cultures in the day - if she can't accept that then she shouldn't be at the wedding.

As for MILs partner - excluding a parent's partner from a wedding, however new, is likely to cause huge fallout. But I do not think I could stomach having an 'active' EDL member at my wedding. Even if he managed to keep his opinions to himself, I'd feel uncomfortable knowing what he was thinking, and let's face it, an 'active' EDL member is unlikely to do that. Especially if he gets a few drinks inside him, he's likely to come out with all kinds of horrible stuff.

Can you stall on the guest list for a while and hope that this guy doesn't stay in MIL's life very long?