My son is now 14, and we went though this at age 7 or there abouts. So long ago I can't remember now! It was before I really knew much about HMS but he was always laying down on the ground when we were out and complaining about walking. I had never used a buggy as I live in a rural area and he was in a backpack until he was nearly 4. Someone had giving us an old buggy which was smelly, in the garage and way too small, but I tried it out one day and it made such a difference that we got a Major on ebay as soon as we could!
Peter then grew, and we bought an Independence. http://www.specialneedspushchairs.co.uk/babyjogger_independence.htm
We thought that it would do until he grew out of his HMS. Oppps!
We have now graduated to a 'real' wheelchair, and it is better in some ways, as he can be independent, but worse as the seating position is more upright in a wheelchair than a buggy.
He has been sitting in it for a lot more than usual this last week as we have been going to lectures at the Cheltenham Science Festival, and he really misses his buggy for that. But on the other hand I can just say 'can you move yourself while I put my jumper on' and he does, which is really helping me a lot.
As for how it was for me, there was a huge learning curve as I had never even used a buggy when he was a baby. I can't tell you the number of times I stood in front of a closed door wondering just how to get through it! But now we are pretty used to it and it has really given us so much more freedom and he gets more exercise too as he knows he can get out any time and walk, knowing that when he is tired he can sit down again.
As for an NHS assessment, we have gone private and had the assessment done at a mobility centre so I can't help you with that.
Oh, and you meet the nicest people when you are with a wheelchair! At things like the Cheltenham Festival you get to sit at the front with the other wheelchairs and you get to know those people! It is like belonging to a secret club where you get to smile and say 'hi' to people that other wise you wouldn't speak to!
Having a wheelchair has been a really positive move for us, and I wish I had done it a lot sooner.
Good luck at the assessment!