Despite what the D2 form says DO NOT SEND YOUR ACTUAL LICENCE BACK TO DVLA. Send a colour photocopy "Because you need the original for car hire purposes". If they screw up and don’t give you all your old entitlements it is the only way you have of proving they were wrong (other than taking the tests again!)
Our experience is that Specsavers often do not understand the difference between car driving and driving a motorhome. The test for "Visual Fields" is different. Check that your Specsaver operator understands the difference e.g. do the test with or without your specs on? One eye at a time or both together? Do your homework by reading up the tests on the DVLA website and take a copy to show the Specsaver operator, just in case. My wife had two Specsaver tests both done incorrectly but her licence was refused anyway. We appealed and DVLA agreed that she could have the same test done by Moorfields Eye Hospital and passed! DVLA now specify the type of test machine and refuse all other machines!
You will also need to get the form D4 from DVLA which is for your medical report. You can ask your GP to complete this for you but get a price first. Mine wanted £250! My wife and I now use a company specialising in completing the medical forms called "D4drivers" - find them on Google typically £50! A UK registered doctor does your form for you while you are there. Locations all over the UK often in a local hotel. You will need to take a recent/current glasses prescription if you need specs for driving. You may still be asked to attend a local Specsavers for further sight tests especially if you have had treatment for eye ailments such as cataracts of Glaucoma.
When you send your D2 and D4 form to DVLA ALWAYS take a full colour photocopy first and then send the original by "First Class, Signed for" service. I made an error on the D2 which implied that I did not need to complete the D4 form. The Post Clerk at DVLA said "Why did he send that in" and just shredded my £250 D4! They eventually sent my new licence without any of the C1/D1/C1E markings. I protested and they eventually agreed to accept the photocopied D4 and eventually issued the correct licence. On another occasion they denied having received the original papers until I told them the date and time they were delivered to DVLA and who signed for them. Suddenly they found them!
Assume that anything that can go wrong, will go wrong, and have a back-up plan!