As Doc described in his excellent answer,
This will almost certainly not cause you any issues
But if you are still nervous about this, you can call the travel agent and ask them to add the MR suffix to the reservation as a name update.
Most airlines have a distinction between name changes (where you rebook the ticket to a different passenger) and name updates (fixing small spelling mistakes or adding a second name, but the passenger is still the same person). Usually name changes incur a change fee, but name updates are free.
This is true for most airlines that are not low-cost, and indeed the BA website tells us that if
the name of a person travelling is spelt wrong or doesn't match their name on their passport: Most spelling mistakes in the name of a traveller can be easily corrected over the phone, when all of the flights on their ticket are operated by British Airways. We don't charge for this type of change, however if the taxes, fees, carrier imposed charges or fuel surcharges on the ticket have changed since the booking was made we'll need to take any additional money due for them at this point.
(Note the catch at the end -- it's up to you if you want to take this risk).
However, note that
If you booked through a travel agent or on a travel website, please contact them directly. We can't change tickets they have issued.
For completeness, as this is not applicable in your case, but for future reference: BA has a 24 hour 'cool off' period in which you can cancel the booking and get a full refund regardless of the reason. So if they get your name completely wrong, immediately contact them.
(Source of the quotes: BA Website)