Ok.
I apologise if it came across rude . I am from South East Kent, in the United Kingdom.
If you could travel as far as East Peckham Rescue, you will find that they offer residential bonding, i.e. your girl stays for several days, will allow her to have time to settle down and she can meet more than one piggy, but not all in one day, which can be stressful and lead to fear-aggressive reactions. It will also mean that when she comes home, the bond will be solid and you do not have to worry about. This is by far the safest way of bonding and worth travelling for in my own experience.
Here is our recommended rescues locator. If it is too far for you, East Peckham may be able to give you tips as to which of your more local rescues may be safe to go to for you and which to beware of. RSPCA centres usually don't do meet&greet as many branches lack experienced volunteers. Each branch is individually run and many are sadly not very small animal savvy when it comes to bonding single piggies.
https://www.theguineapigforum.co.uk/pages/guinea-pig-rescue-locator/
Unfortunately, anybody in this country can call themselves a rescue and the results can be accordingly; there is no licensing and no official control. This is why we have introduced our rescue locator on the top bar; we are listing all rescues whose good standard, practice and experience we can guarantee for.
PS: We also have a recommended piggy savvy vet locator on the top bar.
I hope that this helps you make sure that you have got a good experience and can find a friend of our girl's liking. Age and gender (sow or neutered boar) are by far less important than whether they click or not and whether they can sort out the dominance between themselves. Older sows can sometimes be rather tricky to bond; some are happier on their own. This is one of the reasons why I would strongly recommend to try rescue dating. All you can do is try and see, but ideally in a situation where you do not risk ending up with a rejected piggy on your hands.
PS: I have bonded piggies of all ages, but I have also had my fair share of bondings where two piggies didn't click or could not come to an agreement as to who would come top. Some older sows can on the other hand go overboard with dominance against a younger piggy because they feel insecure. On the other hand, love on first sniff is a wonderful thing to behold - whether that is youngsters, elderlies or piggies with quite an age gap.