I am also a bit intrigued by the idea of providing overwatch for highly mobile tracked recce AFVs by using dismounted ATGM-Teams.
Relocating is slower with such a setup.
As for Spike-NLOS. I am a huge fan of it but it's more of a high precision/high capability artillery support system than something with which one performs AT overwatch.
If one thinks that AT overwatch for recce assets is so important that one puts a specialized vehicle into service one might say that the combination of pure dismount ATGMs with medium range seems a bit strange.
Firefights by overwatch elements tend to be longer ranged.
I can't say exactly how the British Army intends to deploy the ATGM teams, but speaking from the Singaporean experience with the SPIKE...
...you can use these foot-mobile teams for both offensive and defensive missions. It should be pretty obvious for defensive missions - prepare several good firing positions in advance with sufficient cover etc, and co-ordinate with other teams such that as one team fires, the others are reloading/relocating etc.
I've been in exercises where such a set up stalled a simulated OPFOR battalion-sized armoured advance. SPIKE teams were working in concert with SM1s, and it was the ATGM teams that racked up more OPFOR vehicle kills than the light tanks. (Light tanks also suffered several mob/total kills, whilst not a single ATGM team suffered any casualties.) OPFOR took too long to break through, and was subsequently declared dead as enough time was bought for the AH-64s to arrive and mop up.
That was a battalion exercise with IFVs supported by light tanks. With MBTs, I'd think that the OPFOR would have been forced to pull back earlier while causing less casualties to the Blue Force.
In terms of the attack, don't think of using them in a mounted-assault, fire-movement type role. What can be done however is to detach the ATGM teams to the various company commanders, who will deploy them according to his or her will. I have seen aggressive OCs who make the ATGM teams infil with the dismounted infantry so that they can take position as soon as the objective is secured while having some element of surprise. Teams stay in position (i.e. functioning as a Fire Support Group as mentioned by Bonza) until their organic vehicle (often another tracked APC) comes and picks them up, so there really isn't an issue of relocating since once they are set up, they are meant to stay (at least in a localised part of the battlefield), and when they need to exfil, vehicles help them can bug out quick.
Of course, I'm not sure if such a manner of fighting will be relevant to the recce/strike SV, but since there are APC variants out there...I would think that there will be mounted infantry, and consequently the Javelin team will be able to support these dismounts. I don't think there's really a way for dismounted Javelin teams to support the recce/strike SVs when they are advancing (can we just call them 40mm SVs?) and I would think that overwatch for such a battle would be better provided by attack helos, or fixed wing aviation.
Regarding range, I suspect it is a technical thing that involves a trade-off between short flight time (i.e. reduce exposure to passive countermeasures), high hit probability, quick target acquisition, fire-and-forget function...and range. UK might be willing to accept a shorter range for other advantages that the Javelin has, and can bring to the battlefield. In any case, there are many other assets available to the CO at the brigade level to help him take out targets beyond 2.5km...well, I'm boldly assuming that SV-equipped regiments will be supported by AS-90s, as well as aviation assets i.e. AH-64s, but it seems to me to be a fair one.