You cannot use convertible notes if you want the investment to qualify for SEIS or EIS, as HMRC would deem the investors to have the right to get their original capital back.
If you wish to receive cash from investors before your formal SEIS/EIS funding round, you should consider using an Advanced Subscription Agreement.
An Advanced Subscription Agreement is the UKs answer to a SAFE agreement in the United States. It allows investors to invest cash quickly based on stipulated terms when the next funding round occurs.
It is important to note that if you want to qualify for SEIS or EIS, HMRC only allow a 6 month window before your Advanced Subscription Agreement needs to be converted into shares.