nameserver, 123reg re-direct goes down all the time.
If you use URLP you can have the best of both worlds, point the domain A record to the URLP servers, but keep the domain on your own dns, with your own MX records.
If you use sedo you have to change the domain to there nameservers, these are more reliable than a re-direct.. but as you say, you loose the MX record.