MCS, SEG & VAT
Do I need a MCS or Flexi-Orb certified installer?
If you intend to sell your excess energy back to the grid, either a Flexi-Orb or MCS certified installer will have to do your install, register the system and issue you with a certificate, which will be required by your energy supplier if you sign up for SEG.
Make sure you fully understand the role of e.g. the MCS as in essence the MCS can ONLY investigate, arbitrate and potentially withdraw an installer's license to operate if they find it appropriate to do so. MCS don't offer any actual financial protection
The MCS standards for a certified installer can be viewed here where you can get an idea of what to expect from certified installers.
Are their alternatives to MCS or Flexi-orb certified installers?
If you were to delve deeper into the SEG guidance for generators, section 1.12 to 1.14 affirms that there are indeed alternative paths that can be followed to get paid if you can demonstrate your installation and installer are accredited in accordance with EN 45011 or EN ISO/IEC 17065:2012. Suppliers can also exceptionally choose to make payments even if none of these recommendations are satisfied. I hoewer cannot imagine suppliers being as brave despite installations having had to either be notified or approved by DNOs through e.g. qualified electricians.
Good news is that thanks to a DIY'er who has argued a strong case with their supplier, Octopus, a pilot has been put in place to accommodate an alternative route for getting paid for exports. While acceptance is not guaranteed and a hefty adminstrative charge applies, I am aware early indications from participants are that the benefits by far outway the fee given the overall process also ensures their "solar house is in order" with regards to insurances and resale matters.
SEG the FIT replacement
The original FIT and generation incentive payments were subsidies intended to implement a "quick fix" to a government's lagging renewable energy ambitions. It has now been discontinued and replaced with SEG where individuals suppliers set their own rates.
SEG (smart export guarantee) made headlines with initial supplier offerings of 1-2p per kWh vs the over 50p per kWh when FIT was started. Although export pricing per kWh have improved and become more competitive, you may have limited options when combining household and special EV tariffs. It is definitely worth shopping around.
PS - If you have an existing FIT agreement, take care to not impact the current metering with any upgrades of a system as this can see the FIT agreement being terminated.
VAT
New complete PV and storage system purchases and the installation thereof are currently VAT exempt as is the retrofitting of or storage upgrades with installation. Be aware this does not apply to e.g. online purchase.