...Going mad here

I'm attempting to change my flight dates this summer. No longer do I want to come back to Britain on the 28th of July, but I now want to return on the 4th of August, a week later. Ebookers is telling me the flight details can't be changed. I don't understand how it can't be changed. There is still loads of room on the flight, there's no problem with the seats, the only problem is the airline wanting to squeeze as much money out of me as possible by labelling my ticket as fixed.
Eugh. Dealing with these people is beginning to annoy me, especially when they're trying to con me into thinking the call centre is based somewhere in London by offering me 'direct lines' to the operators where the numbers have London codes. Rubbish, all the operators I've spoken to have Indian names, speak with Indian accents and the most crackly phone line you've ever heard. They're not based in Croydon that's for sure. See here and here.
I don't care that they're not in the UK really, but to give a stupid fake number (which still charges me National phone rates so it's not a saving for me) and attempting to sound British just comes across as crass. Now I'm going to have to call me 'dedicated agent' on Monday to try and see if she can sort something out.
This is why people like face-to-face transactions. You at least have an image of someone to get annoyed by, rather than a distant tinny audio clip:

"thank you for choosing eBookers, Europe's number one online travel agency"