Any question answered...

And what stamps work nowadays? I have a vague memory that they changed but picture ones are still usable or something.

If they don't have the barcode they're not valid, as far as I'm aware.

Edit: Sorry, if by pictures you mean other than the HRH mugshot then apparently they are ok.

There's a perspex measuring gauge in Post Offices, which determine small/large letter, parcel etc

I think those dimensions should be on the website.

You can pay to post it online now. You can even get the post man to pick it up from you the next day for free

https://send.royalmail.com/?iid=RM0624_NAV_SEND is good, it'll tell you how much it is based on dimensions/weight.

You can then work out whether you can cover that with stamps.

Large Letter is anything up to 2.5 cm x 25 cm x 35.3 cm. The price depends on the weight:

• up to 100g: £2.50 (£1.55 2nd class)
  
• up to 250g: £3.30 (£1.90 2nd class)
  
• up to 500g: £3.30 (£2.30 2nd class)
  
• up to 750g: £3.30 (£2.50 2nd class)
  
  

1st class stamps are £1.35
2nd class stamps are £0.85
1st class Large Letter are £2.60
2nd class Large Letter are £1.55

As others have said, picture stamps are valid forever but stamps with just the monarch's head on them are only valid if they have a barcode. If you've got a load of old stamps without barcodes you can send them off for replacements: https://www.royalmail.com/sending/barcoded-stamps

My FiL had hundreds of old denomination stamps that he got to swap out.

A lot of the time I just use the royalmail website to print my own postage, stick it to the parcel and then post it locally (if it's small enough to fit into the postbox) or take it to the local post office or sorting office (both of which are half a mile away from me). Arranging the postman collection is unlikely to work for me as they'd probably come at an inconvenient time.