Long post, but I wish I'd known of all this before… Metoclopramide makes me feel a lot better immediately (2-4 hours), then I crash into a really deep sleep of a few hours. Not practical with a todder to care for, so I only use it as a back up/ at weekends. I haven't had any of the other side effects, but I did with prochlorperazine - I think the only way to find out is to try it. Metoclopramide does speed up the emptying of your stomach (into your bowel, not as vomit!) so I feel that it exacerbates the need to eat regularly.
In terms of which medicine they'll prescribe, the GP is limited by what the trust allows them to prescribe. My trust, like many, works on a “traffic light” system - grey is do not prescribe (the trust doesn't fund those drugs), green is any can prescribe (Incl GP, nurse practitioner etc), amber is specialist only (GP can't prescribe, you'd need to see someone in the antenatal department at the hospital for these). Not sure about red because none of the antiemetics were red here…
With a bit of googling, I found the website which lists all the drugs and their traffic light rating in my trust. I printed out the royal college of gynecologists and obstetricians list of recommended medicines (if you Google RCOG green top guidelines no. 69, it's appendix 3), and wrote next to them what their rating was. Then I took the list to the GP and said, “I know you can prescribe x, y, z, I want to try them as per the advice to tackle pregnancy sickness early” (I was 6 weeks and only mildly nauseous at that point so he wasn't keen, but taking them early made a huge difference).
Your GP probably can't prescribe xonvea - I don't think it's funded almost anywhere in the UK. That's because it's recently licensed and so only the brand name is available so it's far, far more costly to the NHS than the alternatives.
The RCOG guidelines have various bits of helpful information in them, including some comments which set my mind at ease re: side effects, and also the fact that many women need 2-3 drugs in combo to effectively manage their nausea. I'm pretty sure that they address the 5 day recommendation and say that if you don't have the side effects in the first five days, you are safe to continue, although that may have been somewhere else… My GP wanted me to swap between metaclopramide and onsansetron, alternating each for five days, but it didn't work because I have to be awake to look after my toddler…
Currently taking cyclizene at night (it makes me fall asleep, but if I take it at night, it helps with the morning's nausea), and onsansetron in the day. I really rate them both, despite cyclizene alone being useless, it's helpful with the onsansetron and if I miss the dose, I vomit the next day. If you do go with onsansetron, I'd 100% push for a laxido prescription and take one sachet for every onsansetron tablet (hard to do when nauseous, but you really need it with onsansetron!)
You could also contact the charity pregnancy sickness support for help and advice. They were quite quick to respond to my WhatsApp
Hope you get something that works for you! I'm sorry, it's so grim isn't it