I started applying for postdoc posts way before submission of my thesis. The earlier you do that, the better.

I waited for two months. In fact, when I was submitting, I had already been told the viva exam date by my supervisor. I submitted in November and defended in January. I packed my thesis and went out to enjoy my Christmas. I only touched my thesis 3 weeks before the viva. It was as if it was written by someone else. It was much easier to spot the mistakes.

As long as you didn't get your PhD from an English speaking country, you are required to have IELTS certificate. They don't care whether your PhD was written in English or not. Let's say, you are from Egypt, you did your PhD in the Netherland (which is not an English speaking country ALTHOUGH courses there are conducted in English), when you get a postdoc post in the UK, you will be required to produce an IELTS certificate. They are very strict on that. If you are thinking of applying for a Postdoc, my advice is to sit for IELTS in advance even if you have not received an offer.

Postdoc should fall under work experience not under qualification.

If she is NOT a national of an English speaking country (listed here >> https://www.gov.uk/tier-2-general/knowledge-of-english), or if her undergrad, MSc or PhD was not obtained from the countries listed in the link I have provided, I am afraid there are no two ways about it. She NEEDS to sit for an English (IELTS) test. The English proficiency will NOT be waived on account of her having previously done some research towards her PhD in the UK. The advice from the international office was spot on; when you come as a short-term research visitor like she did initially, you don't need to have an English test certificate. I am also assuming that her PhD was not awarded by the British university where she spent some time as a research visitor. If I were her, I would straight away start preparing for an IELTS test. Results usually come out after 14 days. It's better to spend £150 on IELTS test than risk losing the exorbitant visa application fees.

No need to ask your potential supervisor just yet. Most supervisors actually push their students to publish and they're expected to provide guidance and support. You will publish papers from your PhD research findings. Once your start your PhD, it's advisable to interact with senior PhD students and postdocs/research associates in your research group. You will learn a lot from them.

When you apply for a postdoc or Lecturer post, recruiters will be looking for a candidate with an impressive publication record. Don't expect to be handed an academic post simply because you were at a Russell group university. If someone from an ex-poly has a good publication record over a Russell graduate, the former will no doubt get the job.

Oh, I have been there. I passed my viva last month with minor corrections. A month later I am still struggling to finish up things which under normal circumstances can be done within a day or two.

In my research group where I am pursuing a PhD, there's a postdoc colleague who got his current post under some interesting circumstances. He attended a conference and a professor in my research group liked his work. The prof approached the guy and asked him if he was willing to take up a postdoc post in his lab. Lucky guy!! The job was advertised formally but of course, the interview was just a formality for him. I am sure there are a number of people who applied for this position and possibly had higher credentials but ended up not getting the job for obvious reasons. Sometimes it's all about networking.

Thanks TreeofLife for an elaborate answer.

It's quite common to see Research Associate posts advertised with the following conditions: "If the PhD will be awarded shortly, the appointment will be made at the Research Assistant level until the PhD has been completed".

I have picked out two keywords in there, "awarded" and "completed". For someone who's had a successful viva, made corrections and handed in the final thesis, but NOT yet graduated, will they be employed as Research Assistant or Research Associate? Many thanks.

Last year, jobs.ac.uk hosted a webinar where they discussed research opportunities in Germany for non-Germans. Three non-German postdocs gave their views about language, culture, work ethics etc. You may find it helpful.
http://www.jobs.ac.uk/careers-advice/working-in-higher-education/2467/postdoctoral-and-research-opportunities-in-germany-google-hangout-on-air-summary

By "switching labs"; do you mean moving to another university or simply moving to another research group within your university? There's no guarantee another lab will "embrace" you fully. It may turn out to be worse, but you have to do a thorough check before making that move.

Five months is too short a period to cultivate a working relationship. Take an initiative to reach out to your colleagues. Target one or two who are seemingly friendly to you, say hi to them every morning. Talk about your interests, sports, movies, your culture, global politics, language etc. Whenever there are social events, such as the end of year dinner for the lab, don't hesitate to take part. If someone invites you for a cup of coffee, don't shy away. It may turn out that your colleagues view you as someone who is reclusive, shy and difficult to engage in a conversation with. You should also seek help from laboratory technicians and other support staff on certain techniques.