You will be able to check through your luggage at MAN. You can also get your bps if you have not already got them at home when OLCI opens.

If arriving at LHR in F BA metal then you can use the CCR before your MAN flight, keep hold of your inbound bp as you will need to show it.

As you are on separate tickets you will have no protection is you misconnect. BA will probably sort you out but they are not obliged to.

Is this a reward booking? You can add the domestic in later and I have done it myself (subject to availability coming up of course). There would be no extra avios due but you will need to pay the extra carrier surcharges to add it in (i.e. what you would have paid originally had you been able to include it). From experience LBA was around £70-80 so expect something similar.