If there is a spare MCB slot you can add SPD in a small DIN box outside of an older DB if really needed.
For better protection you can route the sub-main through the DIN box to the SPD and then to the DB, assuming the type of SPD is good to the supply protection (e.g. 100A fuse). Many of the cheaper SPD need an MCB in the 32A or so region for protection, but not all. For example the Axiom single-unit from Toolstation, etc, is good to a 100A fuse and only ÂŁ20!
• Single pole and neutral• Single module• Does not require MCB protection• DIN-rail mountable• Type 2, single phase, double pole, suitable for TN-S, TN-C-S and TT earthing• Suitable for use in Axiom consumer units• Complies with IEC 61643-11• Dimensions: 18 x 81 x 71mm• 1 year warrantyOver...
www.toolstation.com
The line terminal will take (probably) 2 * 16mm but neutral won't, so it needs a box with a neutral bar if used in this manner.
Also an external SPD need not match the DB brand as it is not fitting in to the busbar design, etc.
RCD protection is far more important, so that is good to hear!