"Dear ManagerName,
Please accept this email as notice of my resignation from position as Admin Support. As I don't have any holiday booked or owed to me, I think my last day will be DD.mm.yyyy, but I would be grateful if you could confirm this.
It has been a pleasure working for you, and I have especially enjoyed/will always remember/am grateful for the opportunities to [insert mention of something nice about the team, the work, the management, the progress made while you worked there, some group achievement, the amazing work-life balance, or whatever].
I am excited about pursuing a new job opportunity, but of course I am sad to be leaving the team here. I look forward to ensuring a smooth handover to my successor.
Although I'm currently isolating at home, please feel free to contact me on the number below during working hours, if you would like to discuss this further.
Kind regards,
4Cats1dog
Tel. #....."
[Unless you want to open a negotiation to get more hours and more pay in your current job, I simply wouldnt mention anything more about the new job in the letter. You're leaving, end of. If your manager asks, then say you've been thinking it is time for a change and you are keen to increase your hours and pay also, so the new job was hard to refuse as the long-term prospects are very positive. You don't need to feel guilty - it's business, and they will replace you. Also I wouldn't worry about being in isolation - that's not your fault. When your manager calls, mention you are sorry to email while you're off work but didnt want to leave it another week. If you are not actually sick, you could offer to do some wfh to help with handover or keep on top of work so company isnt falling behind. You could also offer to increase your hours to 5 days a week part time, to help train a new person, in your last few weeks. But only if you want to.]