Upgrading Shimano Groupset

[SPritc80]

My place of work are looking at
doing a charity ride from Glasgow to Stockton-on-Tees in September. I need
a little advice. I have a 5 year old, Pinnacle Dolomite 3. I am looking to
upgrade the groupset to Shimano 105, which will upgrade from 9 to 11 speed,
but I can’t find if the Alexrims Race24 hub will take the Upgrade or will
I need new rims as well? My main reason for upgrading is due to the age of
the groupset, my lack of maintenance (should have done better) and the 200
miles over 3 days. Not to mention the next 3 months of training it is going
to take. The gears seem smooth at the moment. I am such a couple of extra
gears for the Scottish hills can’t hurt. Thank you for any advice you can
give.

 

[DCLane]

9 to 11 speed is fine, it'll give you a few more gears that's all. 9 speed is fine otherwise.

You can check if the wheels will take 11 speed by removing the current cassette. If '11 speed' isn't printed on it then it's unlikely to take an 11 speed cassette, just 10 speed at the most.

 

[Ajax Bay]

My main reason for upgrading is due to the age of
the groupset, . . . The gears seem smooth at the moment.

Pinnacle/dolomite-3
Surely the 'groupset' is only 5 or 6 years old. At this distance from a ride in September you are better getting on your bike, enjoying the smooth changing your STIs (assume Sora) offer and don't disrupt things by seeking to upgrade. Don't upgrade: ride up grades (as I think the saying goes). At least don't kid yourself that your motive for upgrading is for the reasons described.
Procuring (current) 105 STIs, FD, RD, new rear wheel (with 11sp freehub, ?front wheel to 'match'), cassette and chain will cost more than the bike's worth - it's your money of course.
As @DCLane says, 11sp will give you no better low gear. A wider range cassette (probably 12-27 there at present) say 11-30 will give you that and the Sora RD will handle that without missing a beat (NB new chain to 'fit' new cassette).
The best 'upgrades' (if one's needed besides a larger large sprocket) are good tyres and brake blocks, without forgetting a full replacement of the cables, inner and outer (given the confessed "lack of maintenance").

 

[DRM]

What's been written above basically, I think a new chain, cassette with a wider gear, and new cable inners & outers(Brake & Gear) along with quality brake blocks will transform your bike, there's nothing wrong whatsoever with Sora, check your tyres are in good condition and get some riding done