This is not an optimal solution, because there will be a long delay after dialing before the SPA sends the call to the PBX. Instead, you can construct your dial plan so most calls have no delay and some only a short delay.
An SPA dial plan consists of several patterns. As you dial, the device attempts to match the digits you enter against the patterns. If there are only partial matches (you haven’t provided all the digits for any pattern), the system waits the long timer to allow you to enter more. If there is a complete match and all other patterns have mismatched, the device knows your number is complete and sends it to the PBX immediately. If there is a complete match but one or more partial matches, your number may be valid but may also be the start of a valid longer number; the device waits for only the short timer before sending the number. If everything mismatches, the device plays a ‘reorder’ tone and does not send the call to the PBX.
Assuming that your system does not require you to dial 0 for an outside line (recommended for several reasons), here is a guess at a suitable dial plan:
(999S0|1[1-3]x|1[45]xx|08001111|0845464x|0[58]00xxxxxx|01[2-9][02-9]xxxxxx|0[1-9]xxxxxxxxx|[2-9]xxxxx|00xxxxxxxx.)
I assumed here that you have no more than 10 extensions, numbered 130-139. You could of course choose other ranges that don’t conflict with the UK numbering plan.
My guides were Telephone numbers in the United Kingdom - Wikipedia and https://www.area-codes.org.uk/01483.php – I don’t know whether they are accurate. Let me know of any errors.
BTW, did you get a SIP trunk set up?