Letter of employment of the British standard visitor visa

I am a Chinese citizen who is planning to apply for a British visitor visa to China to attend a conference in the UK in July 2019. I have just completed my first work contract, which is in an American university, and I will be an instructor. at another American university in August 2019. The moment I apply for a visa for the UK and attend the conference is the gap between my end of my first job and the start of my second job. Can I provide the employment letter of my future employer? Since I apply for a visa in China, must the employment letter come from a Chinese employer, even if I am a Chinese citizen, I have my own savings and a permanent home with my parents?