To add an SPF record to your 123-reg domain name, please do the following:
Firstly, you need to ensure your domain is pointed to our nameservers:
If the domain is not pointed to the 123-reg nameservers, you will be unable to manage them from the 123-reg control panel.
For more information on changing your nameservers please view the following article:
How do I change the nameservers for my domain name?
You will need to generate the record first this can be done at Microsoft's SPF record wizard.
Once you have the record, you will need to apply it to your domain name, this is done in the 123-reg control panel.
In the Domain names section, select the relevant domain name in the drop-down list and click on the Manage button.
Scroll down to the Advanced Domain Settings section and click on the Manage DNS (A, MX, CNAME, TXT) link.
From the Advanced DNS tab.
Select TXT/SPF from the Type drop down list in the Add new entry section.
In the Hostname field enter an @ symbol.
In the Destination TXT/SPF field enter your SPF record.
Click on the Add new entry button to save the new record.