im sure the challi will still be a part of the British army plans in the future.Having something like a MGS stryker would definately mean a whole host of problems likely to deal with armor issues and the stress of firing the main gun.
Such vehicles are more likely to succumb to heavy armor fire if fired upon and i see it the way they could be used is to support challi tanks as a fire support based weap like waylander said
The dilemma the UK has to face is budget restrictions combined with the need to upgrade many legacy items. I doubt this will allow for both wheeled and tracked direct fire vehicles.
Fortunately UOR's have delivered a very large quantity of wheeled MRAP's of all shapes and sizes, some fitted for/with Javelin firing posts/GMG/.50 etc. This relatively new fleet can support light infantry peace keeping operations and low intensity conflict. Whilst they do not have the off-road /armoured abilities of the Striker family they can fulfill a useful role in environments where IED's and light weapons are in the majority.
The UK armoured corps over the last 20 odd years has supported heavy armour with tracked recce assets, hence the desire to move forward with FRES SV as a replacement for CVRT. Adding to that a wheeled Striker type variant will stretch the budget to breaking point, I can't see it happening under the current climate when the Warrior upgrade programme is also a priority.
The future plan is for 5 x formation recce regiments to be equipped with 2 x Sqn of FRES SV and 1 x Sqn of Jackal and 5 x Challenger II armoured regiments (227 tanks in total). The UK has kicked into the long grass the idea of medium level units equipped with a new generation of wheeled AFV's and fire support variants. Any spare cash can be used to stick a direct fire turret on an ASCOD chassis. A formation recce troop could then consist of 1 x direct fire and 2 x 40mm CTA equipped vehicels.
I even believe the A400 transport requirement has been ditched because of lessons learnt in Afghanistan and the C17 factor, the latter being able to lift all the current range of UK tracked vehicles.
Instead of Heavy, Medium and Light formations, the UK will end up with basically heavy (tracked) and light (wheeled) mixed together in five Brigades with one ready to deploy almost immediately to back-up 16AA and/or 3 Commando in an emergency.
IMHO when it comes to replacing the Challie II it will either be a US/UK partnership or Anglo-French partnership. The former for obvious reasons the latter as a result of the increasing closeness driven by the need to mirror both armies shared capabilties for future joint ops (Libya type or more ground unit intensive). 40mm CTA will become standard in both armies, so why not a standardised future MBT based on a 140mm CTA?