Don't count on getting what your rights are, as most airlines will flat out refuse to oblige. Because it is so darn near impossible to get legislated compensation (and beware; this is NOT denied boarding, this is delay. Same goes for rules etc. etc. that apply), at our Dutch side of the Channel it is very common to let your claim be handled by a company that specialises in it for a percentage of it. Biggest advantage is that they can get to official information like why a flight was delayed or cancelled, where you as a passenger can not. Many have been told in the past "weather" (no compensation) when it would turn out to be for very different reasons (and right to compensation). Even they commenly fight for months or years before things are properly taken care off. By the way, this does not just limit to EU-based airlines but ANY and ALL flights departing from, arriving in and connecting within the EU and excludes delays caused by weather, disasters (remember that vulcano?

etc. etc. )
Because of this being such a problem of "customer not being able to get their rights", there are major lawsuits ongoing about this. Not that up to par about other countries, but do know that here we've long since passed Dutch court and it has been taken on at a EU level long since. All judgements since have been in passenger favours, but again; airlines are big and refuse to cough up, simply put. Starting to get annoying to all kinds of court options we have in the EU and EU parlement alike. It is under the attention to change things further so if need be airlines can be factually be forced (read; huge fines) to uphold and make it easier for customers to claim and be succesfull themselves. Until then it is hit or miss when it comes to airlines respecting legislation or not. Do know of a US based carrier that did pro-actively take care of this when having cancelled a TA flight last year, but can't remember which one it was right now. It's such a hot button subject here that it is one of the continuing items on consumer right programs, which has been a catalyst for getting certain EU consumer organisations involved, setting up the big law suits (to no or very little costs for the consumer when adding onto) etcetc. Time will tell.
Travel assurance is something that has little to do with this. It's a different subject, where one does not have to bite the other. Not enough into the subject when it comes to UK insurance companies, but when stating "compensation will only be given when no other compensation was entitled" with most -if not all?- Dutch relates to costs like food, shelter etc. Only the daily standard compensation for "lost day of vacation" might sometimes be lowered by monetary compensation given by airlines. Given the time it tends to take in most cases to get entitled compensation, most travel insurances here just pay out the (very low) daily compensation and do not come asking for that amount back when years later a claim finally gets dealt with.
Did a very quick google, but sure enough; first name was a hit. EUclaim also has a branch in the UK. It's one of those companies that takes over a claim for a percentage or set fee (after getting success). Didn't look any beyond but both them and euroclaim are very commonly used here by Dutch passengers and most are positive. Not saying you should use them (you are an adult that can make your own decisions), but they can be a good place to start for getting right information about rights and how easy or difficult it can be.