Early retiree returning to work part time as sole trader

I have taken early retirement over 2 years ago and receiving my pension. I am considering returning to work on an as required basis a max. of 2 days a week to help keep my brain active mainly. I believe the best way for me to do this would be to register as a sole trader and fill out a tax return for the years I continue to work. I think this is the easiest method because I will not work on the weeks I am not required. I want to return on my terms primarily and be able to walk away if I feel I do not want to do it anymore. The company I would be engaged via would prefer me to go under their umbrella company but I am concerned that this would then put me on some kind of emergency tax codes etc and affect my pension payment considerably. As I will be working on an adhoc when required basis I am quite happy to go down the sole trader method. I think this also satisfies the IR35 rules also. Would this be an acceptable method to HMRC? Regards. John.

Hi,
While we are unable to provide specific advice tailored to individual circumstances, we can confirm that both routes are acceptable to HMRC. 
Thank you.