Argos Reservations now work but it took me a while to figure it out
Go to Argos website and search for 'iPad 16GB' or 32GB or 64GB remember to include the ' in the search box or the New iPad results wont come up! If you only search for iPad you only get the holding advert page.
Note down the catalogue number and text the <catalogue number> <store number> to 61201. If its out of stock it will suggest a near by store that has stock, then just reply with reserve
Use the store locator to find the store numbers, good luck!
EDIT - Here's the catalogue numbers so is easier
New iPad with WiFi 16GB - Black. 5087444
New iPad with WiFi 16GB - White. 5087451
New iPad with WiFi 32GB - Black. 5087468
New iPad with WiFi 32GB - White. 5087475
New iPad with WiFi 64GB - Black. 5087482
New iPad with WiFi 64GB - White. 5087499
New iPad with WiFi + 4G 16GB - White. 5087516
New iPad with WiFi + 4G 16GB - Black. 5087509
New iPad with WiFi + 4G 32GB - White. 5087530
New iPad with WiFi + 4G 32GB - Black. 5087523
New iPad with WiFi + 4G 64GB - White. 5087554
New iPad with WiFi + 4G 64GB - Black. 5087547